diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/ScriptAnalysis/ScriptAnalysisService.cs b/src/ZB.MOM.WW.ScadaBridge.CentralUI/ScriptAnalysis/ScriptAnalysisService.cs
index 5157ed09..b36f106a 100644
--- a/src/ZB.MOM.WW.ScadaBridge.CentralUI/ScriptAnalysis/ScriptAnalysisService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/ScriptAnalysis/ScriptAnalysisService.cs
@@ -177,6 +177,15 @@ public class ScriptAnalysisService
     private const int SandboxMaxConsoleChars = 32_000;
     private const int SandboxMaxReturnJsonChars = 32_000;
 
+    /// <summary>
+    /// Shared options for the per-Test-Run return-value JSON serialize in
+    /// <see cref="SerializeReturn"/> (perf remediation, arch-review WP1.5) — avoids
+    /// allocating a new <see cref="JsonSerializerOptions"/> per run. Settings preserved
+    /// exactly.
+    /// </summary>
+    private static readonly JsonSerializerOptions SandboxReturnJsonOptions =
+        new() { WriteIndented = true };
+
     private const int SandboxMaxCallSharedDepth = 16;
 
     /// <summary>
@@ -520,7 +529,7 @@ public class ScriptAnalysisService
         var typeName = value.GetType().Name;
         try
         {
-            var json = JsonSerializer.Serialize(value, new JsonSerializerOptions { WriteIndented = true });
+            var json = JsonSerializer.Serialize(value, SandboxReturnJsonOptions);
             if (json.Length > SandboxMaxReturnJsonChars)
                 json = json[..SandboxMaxReturnJsonChars] + "\n… (truncated)";
             return (json, typeName);
diff --git a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
index 850ed1f9..ac91678f 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
@@ -27,6 +27,15 @@ public class ExternalSystemClient : IExternalSystemClient
     private readonly ILogger<ExternalSystemClient> _logger;
     private readonly ExternalSystemGatewayOptions _options;
 
+    /// <summary>
+    /// Shared options for the per-call AuthConfiguration JSON parse in
+    /// <see cref="TryParseJsonAuth"/> (perf remediation, arch-review WP1.5) — avoids
+    /// allocating a new <see cref="JsonSerializerOptions"/> (and its internal cached
+    /// metadata) on every outbound call. Settings preserved exactly.
+    /// </summary>
+    private static readonly JsonSerializerOptions AuthJsonOptions =
+        new() { PropertyNameCaseInsensitive = true };
+
     /// <summary>
     /// Initializes a new instance of the ExternalSystemClient.
     /// </summary>
@@ -755,7 +764,7 @@ public class ExternalSystemClient : IExternalSystemClient
         {
             fields = JsonSerializer.Deserialize<Dictionary<string, string?>>(
                 config,
-                new JsonSerializerOptions { PropertyNameCaseInsensitive = true })
+                AuthJsonOptions)
                 ?? new Dictionary<string, string?>(StringComparer.OrdinalIgnoreCase);
 
             // Normalize to case-insensitive so "Header"/"header" both resolve.
diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementEndpoints.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementEndpoints.cs
index 9c44f042..033a0504 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementEndpoints.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementEndpoints.cs
@@ -17,6 +17,15 @@ public static class ManagementEndpoints
 {
     private static readonly TimeSpan DefaultAskTimeout = TimeSpan.FromSeconds(30);
 
+    /// <summary>
+    /// Shared options for the per-request command payload JSON parse in
+    /// <see cref="ParseCommand"/> (perf remediation, arch-review WP1.5) — avoids
+    /// allocating a new <see cref="JsonSerializerOptions"/> per inbound management
+    /// request. Settings preserved exactly.
+    /// </summary>
+    private static readonly JsonSerializerOptions CommandPayloadJsonOptions =
+        new() { PropertyNameCaseInsensitive = true };
+
     /// <summary>
     /// Resolves the ManagementActor Ask timeout from configuration.
     /// Falls back to <see cref="DefaultAskTimeout"/>
@@ -197,7 +206,7 @@ public static class ManagementEndpoints
                 ? p.GetRawText()
                 : "{}";
             var command = JsonSerializer.Deserialize(payloadJson, commandType,
-                new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;
+                CommandPayloadJsonOptions)!;
             return CommandParseResult.Ok(command);
         }
         catch (Exception ex)
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
index aeed7315..8285bd41 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
@@ -7,6 +7,7 @@ using ZB.MOM.WW.ScadaBridge.Commons.Messages.DebugView;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Instance;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.ScriptExecution;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Streaming;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Protocol;
 using ZB.MOM.WW.ScadaBridge.Commons.Types;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Flattening;
@@ -92,8 +93,51 @@ public class InstanceActor : ReceiveActor
     // attribute lookup must be O(1) rather than a linear scan of
     // _configuration.Attributes. Built once in the constructor from the
     // deserialized configuration (last-wins on duplicate canonical names,
-    // mirroring the rest of the actor's by-name dictionaries).
-    private readonly Dictionary<string, ResolvedAttribute> _resolvedAttributeByName = new();
+    // mirroring the rest of the actor's by-name dictionaries). Each entry
+    // additionally carries its DataType/ElementDataType parsed ONCE at
+    // config-apply time (perf remediation, arch-review WP1.5) so the hot
+    // TagValueUpdate path never re-Enum.TryParse's a string per update.
+    private readonly Dictionary<string, AttributeIndexEntry> _resolvedAttributeByName = new();
+
+    // Static-override write coalescing (perf remediation, arch-review WP1.5):
+    // mirrors NativeAlarmActor's MarkDirtyUpsert/FlushDirtyUpserts (P4) shape.
+    // A burst of rapid SetAttribute calls on the same or different attributes
+    // collapses into one batched SQLite transaction on a short single-shot
+    // timer instead of a write per call. Keyed by attribute name — only the
+    // LATEST value per attribute survives a coalescing window, which matches
+    // the "in-memory state is authoritative" fire-and-forget persistence model
+    // already documented on the single-write path this replaces.
+    private readonly Dictionary<string, string> _dirtyStaticOverrides = new();
+    private ICancelable? _staticOverrideFlushTimer;
+    private readonly TimeSpan _staticOverrideFlushInterval;
+
+    /// <summary>
+    /// Cached string forms of <see cref="QualityCode"/> (perf remediation, arch-review
+    /// WP1.5) — the TagValueUpdate ingest path calls this once or twice per update, so a
+    /// static lookup replaces repeated <see cref="Enum.ToString()"/> reflection.
+    /// </summary>
+    private static readonly Dictionary<QualityCode, string> QualityStrings = new()
+    {
+        [QualityCode.Good] = "Good",
+        [QualityCode.Bad] = "Bad",
+        [QualityCode.Uncertain] = "Uncertain",
+    };
+
+    private static string QualityToString(QualityCode quality) =>
+        QualityStrings.TryGetValue(quality, out var s) ? s : quality.ToString();
+
+    /// <summary>
+    /// O(1) attribute-resolution index entry (perf remediation, arch-review WP1.5): wraps a
+    /// <see cref="ResolvedAttribute"/> together with its <see cref="DataType"/> /
+    /// element <see cref="DataType"/> / IsList classification, parsed ONCE when the entry
+    /// is built (constructor) instead of re-parsed via <see cref="Enum.TryParse{DataType}"/>
+    /// on every access.
+    /// </summary>
+    private sealed record AttributeIndexEntry(
+        ResolvedAttribute Attribute,
+        DataType DataType,
+        DataType? ElementDataType,
+        bool IsList);
 
     // WaitForAttribute (spec §4.2): one-shot waiter registry keyed by the
     // request CorrelationId. Each entry holds the watched attribute name, the
@@ -149,7 +193,8 @@ public class InstanceActor : ReceiveActor
         ILogger logger,
         IActorRef? dclManager = null,
         ISiteHealthCollector? healthCollector = null,
-        IServiceProvider? serviceProvider = null)
+        IServiceProvider? serviceProvider = null,
+        TimeSpan? staticOverrideFlushInterval = null)
     {
         _instanceUniqueName = instanceUniqueName;
         _storage = storage;
@@ -161,6 +206,8 @@ public class InstanceActor : ReceiveActor
         _dclManager = dclManager;
         _healthCollector = healthCollector;
         _serviceProvider = serviceProvider;
+        // Defaults to 100 ms, mirroring NativeAlarmActor's persistFlushInterval (P4).
+        _staticOverrideFlushInterval = staticOverrideFlushInterval ?? TimeSpan.FromMilliseconds(100);
 
         // Deserialize the flattened configuration
         _configuration = JsonSerializer.Deserialize<FlattenedConfiguration>(configJson);
@@ -172,18 +219,23 @@ public class InstanceActor : ReceiveActor
         {
             foreach (var attr in _configuration.Attributes)
             {
-                // Index resolved attributes for O(1) lookup on the hot
-                // TagValueUpdate ingest path (last-wins on duplicate names).
-                _resolvedAttributeByName[attr.CanonicalName] = attr;
+                // Parse DataType/ElementDataType ONCE here (config-apply time) and index
+                // the wrapping entry for O(1) lookup on the hot TagValueUpdate ingest path
+                // (last-wins on duplicate names) — perf remediation, arch-review WP1.5.
+                var dataType = ParseDataType(attr.DataType);
+                var elementDataType = ParseElementDataType(attr.ElementDataType);
+                var isList = dataType == DataType.List;
+                _resolvedAttributeByName[attr.CanonicalName] =
+                    new AttributeIndexEntry(attr, dataType, elementDataType, isList);
 
                 // A STATIC List attribute's default is the canonical JSON
                 // array string. Decode it to a typed List<T> for in-memory reads
                 // so scripts see a real collection. Scalars store their raw
                 // string unchanged. A malformed List default decodes to null and
                 // is marked Bad quality rather than crashing the actor.
-                if (IsListAttribute(attr))
+                if (isList)
                 {
-                    var decoded = DecodeAttributeValue(attr, attr.Value);
+                    var decoded = DecodeAttributeValue(dataType, elementDataType, attr.Value, attr.CanonicalName);
                     _attributes[attr.CanonicalName] = decoded;
                     _attributeQualities[attr.CanonicalName] =
                         decoded is null && !string.IsNullOrEmpty(attr.Value) ? "Bad"
@@ -257,6 +309,10 @@ public class InstanceActor : ReceiveActor
 
         // Handle internal messages
         Receive<LoadOverridesResult>(HandleOverridesLoaded);
+
+        // Coalesced flush of buffered static-override writes (P4 pattern, see
+        // NativeAlarmActor.FlushDirtyUpserts).
+        Receive<FlushStaticOverrides>(_ => FlushDirtyStaticOverrides());
     }
 
     /// <inheritdoc />
@@ -300,6 +356,13 @@ public class InstanceActor : ReceiveActor
             timer.Cancel();
         _tagSubscribeRetryTimers.Clear();
 
+        // Best-effort final flush of any coalesced static-override writes so a graceful
+        // stop does not lose the last <= flush-interval of writes (P4 pattern, mirrors
+        // NativeAlarmActor.PostStop). Fire-and-forget — the in-memory attribute value
+        // (already applied) remains authoritative even if this final persist is lost.
+        _staticOverrideFlushTimer?.Cancel();
+        FlushDirtyStaticOverrides();
+
         // Operational `instance_lifecycle` event — instance stopped. An
         // instance stops on disable, delete, redeployment, and graceful shutdown;
         // this single point covers them all.
@@ -369,9 +432,9 @@ public class InstanceActor : ReceiveActor
     /// </summary>
     private void HandleSetStaticAttribute(SetStaticAttributeCommand command)
     {
-        // Resolve the target attribute's data binding from the flattened config.
-        var resolved = _configuration?.Attributes
-            .FirstOrDefault(a => a.CanonicalName == command.AttributeName);
+        // O(1) lookup via the existing hot-path index instead of a linear scan over
+        // _configuration.Attributes (perf remediation, arch-review WP1.5).
+        _resolvedAttributeByName.TryGetValue(command.AttributeName, out var resolved);
 
         // Reject writes targeting an attribute that does not exist
         // on the deployed instance. Without this check, an inbound API
@@ -398,8 +461,8 @@ public class InstanceActor : ReceiveActor
         }
 
         var isDataSourced =
-            !string.IsNullOrEmpty(resolved.DataSourceReference)
-            && !string.IsNullOrEmpty(resolved.BoundDataConnectionName);
+            !string.IsNullOrEmpty(resolved.Attribute.DataSourceReference)
+            && !string.IsNullOrEmpty(resolved.Attribute.BoundDataConnectionName);
 
         if (isDataSourced)
         {
@@ -423,7 +486,7 @@ public class InstanceActor : ReceiveActor
         // store the string verbatim. (HandleSetStaticAttribute already rejected
         // unknown attributes, so resolved is non-null here, but guard defensively.)
         if (_resolvedAttributeByName.TryGetValue(command.AttributeName, out var resolved)
-            && IsListAttribute(resolved))
+            && resolved.IsList)
         {
             // The script path pre-encodes valid canonical JSON via ScopeAccessors,
             // but the Inbound API / direct-command path can submit an arbitrary
@@ -435,7 +498,8 @@ public class InstanceActor : ReceiveActor
             // (valid — clearing) AND a malformed non-empty input (invalid). Only the
             // latter is rejected, hence the explicit IsNullOrWhiteSpace guard. An empty
             // list "[]" decodes to a non-null empty List, so it passes through.
-            var decoded = DecodeAttributeValue(resolved, command.Value);
+            var decoded = DecodeAttributeValue(
+                resolved.DataType, resolved.ElementDataType, command.Value, command.AttributeName);
             if (!string.IsNullOrWhiteSpace(command.Value) && decoded == null)
             {
                 _logger.LogWarning(
@@ -474,32 +538,67 @@ public class InstanceActor : ReceiveActor
 
         PublishAndNotifyChildren(changed);
 
-        // Persist asynchronously -- fire and forget since the actor is the source of truth.
-        var instanceName = _instanceUniqueName;
-        var attributeName = command.AttributeName;
-        var logger = _logger;
-        _storage.SetStaticOverrideAsync(_instanceUniqueName, command.AttributeName, command.Value)
-            .ContinueWith(t =>
-            {
-                logger.LogWarning(
-                    t.Exception?.GetBaseException(),
-                    "Failed to persist static override for {Instance}.{Attribute}; in-memory state is authoritative",
-                    instanceName,
-                    attributeName);
-            }, TaskContinuationOptions.OnlyOnFaulted);
+        // Coalesce the persist: record the latest value and arm the short batched-flush
+        // timer (P4 pattern) instead of firing a SQLite write per call — the actor's
+        // in-memory state (already updated above) remains the source of truth regardless
+        // of when the buffered write lands.
+        MarkDirtyStaticOverride(command.AttributeName, command.Value);
 
         Sender.Tell(new SetStaticAttributeResponse(
             command.CorrelationId, _instanceUniqueName, command.AttributeName,
             true, null, DateTimeOffset.UtcNow));
     }
 
+    /// <summary>
+    /// Records the latest value per attribute name and arms the coalesced flush timer
+    /// (perf remediation, arch-review WP1.5) — mirrors NativeAlarmActor's
+    /// MarkDirtyUpsert/FlushDirtyUpserts (P4) shape. A burst of rapid static writes
+    /// collapses into one batched SQLite transaction instead of a write per call.
+    /// </summary>
+    private void MarkDirtyStaticOverride(string attributeName, string value)
+    {
+        _dirtyStaticOverrides[attributeName] = value;
+        if (_staticOverrideFlushTimer == null)
+        {
+            _staticOverrideFlushTimer = Context.System.Scheduler.ScheduleTellOnceCancelable(
+                _staticOverrideFlushInterval, Self, FlushStaticOverrides.Instance, Self);
+        }
+    }
+
+    /// <summary>
+    /// Persists all coalesced static-override writes in one batched transaction (P4),
+    /// then disarms the flush timer. Fire-and-forget with OnlyOnFaulted logging — a
+    /// failed write never blocks the actor; the in-memory attribute value (already
+    /// applied) remains authoritative.
+    /// </summary>
+    private void FlushDirtyStaticOverrides()
+    {
+        _staticOverrideFlushTimer = null;
+        if (_dirtyStaticOverrides.Count == 0)
+            return;
+
+        var batch = _dirtyStaticOverrides
+            .Select(kv => (AttributeName: kv.Key, Value: kv.Value))
+            .ToList();
+        _dirtyStaticOverrides.Clear();
+
+        var instanceName = _instanceUniqueName;
+        var logger = _logger;
+        _storage.SetStaticOverridesAsync(instanceName, batch)
+            .ContinueWith(t => logger.LogWarning(
+                t.Exception?.GetBaseException(),
+                "Failed to persist {Count} static overrides for {Instance}; in-memory state is authoritative",
+                batch.Count, instanceName),
+                TaskContinuationOptions.OnlyOnFaulted);
+    }
+
     /// <summary>
     /// Data-sourced attribute write: forwards a write request to the DCL and pipes
     /// the device write result back to the caller. The in-memory value is left
     /// untouched (it is refreshed by the subscription when the device confirms);
     /// no static override is persisted for a data-sourced attribute.
     /// </summary>
-    private void HandleSetDataAttribute(SetStaticAttributeCommand command, ResolvedAttribute resolved)
+    private void HandleSetDataAttribute(SetStaticAttributeCommand command, AttributeIndexEntry resolved)
     {
         var caller = Sender;
         var correlationId = command.CorrelationId;
@@ -526,9 +625,10 @@ public class InstanceActor : ReceiveActor
         // write rather than forward garbage to the device (mirrors the static-path
         // rejection in HandleSetStaticAttributeCore). Scalars are unchanged.
         object? writeValue = command.Value;
-        if (IsListAttribute(resolved) && !string.IsNullOrWhiteSpace(command.Value))
+        if (resolved.IsList && !string.IsNullOrWhiteSpace(command.Value))
         {
-            var decoded = DecodeAttributeValue(resolved, command.Value);
+            var decoded = DecodeAttributeValue(
+                resolved.DataType, resolved.ElementDataType, command.Value, attributeName);
             if (decoded == null)
             {
                 _logger.LogWarning(
@@ -545,8 +645,8 @@ public class InstanceActor : ReceiveActor
 
         var writeRequest = new WriteTagRequest(
             correlationId,
-            resolved.BoundDataConnectionName!,
-            resolved.DataSourceReference!,
+            resolved.Attribute.BoundDataConnectionName!,
+            resolved.Attribute.DataSourceReference!,
             writeValue,
             DateTimeOffset.UtcNow);
 
@@ -598,8 +698,8 @@ public class InstanceActor : ReceiveActor
         foreach (var kv in request.AttributeEncodedValues)
         {
             if (!_resolvedAttributeByName.TryGetValue(kv.Key, out var resolved)
-                || string.IsNullOrEmpty(resolved.DataSourceReference)
-                || string.IsNullOrEmpty(resolved.BoundDataConnectionName))
+                || string.IsNullOrEmpty(resolved.Attribute.DataSourceReference)
+                || string.IsNullOrEmpty(resolved.Attribute.BoundDataConnectionName))
             {
                 caller.Tell(new WriteAttributeBatchResponse(
                     cid, false, $"Attribute '{kv.Key}' is not a data-sourced attribute."));
@@ -607,8 +707,8 @@ public class InstanceActor : ReceiveActor
             }
 
             if (connName == null)
-                connName = resolved.BoundDataConnectionName;
-            else if (!string.Equals(connName, resolved.BoundDataConnectionName, StringComparison.Ordinal))
+                connName = resolved.Attribute.BoundDataConnectionName;
+            else if (!string.Equals(connName, resolved.Attribute.BoundDataConnectionName, StringComparison.Ordinal))
             {
                 caller.Tell(new WriteAttributeBatchResponse(
                     cid, false, "Batch write spans multiple data connections; not supported."));
@@ -619,9 +719,10 @@ public class InstanceActor : ReceiveActor
             // MV: a data-sourced List attribute's encoded value is the canonical JSON
             // array string — decode it to a typed List<T> so the DCL/Variant write
             // produces a real array (same poison-rejection rule as HandleSetDataAttribute).
-            if (IsListAttribute(resolved) && !string.IsNullOrWhiteSpace(kv.Value))
+            if (resolved.IsList && !string.IsNullOrWhiteSpace(kv.Value))
             {
-                var decoded = DecodeAttributeValue(resolved, kv.Value);
+                var decoded = DecodeAttributeValue(
+                    resolved.DataType, resolved.ElementDataType, kv.Value, kv.Key);
                 if (decoded == null)
                 {
                     caller.Tell(new WriteAttributeBatchResponse(
@@ -631,7 +732,7 @@ public class InstanceActor : ReceiveActor
                 wv = decoded;
             }
 
-            values[resolved.DataSourceReference!] = wv;
+            values[resolved.Attribute.DataSourceReference!] = wv;
         }
 
         string? triggerPath = null;
@@ -639,23 +740,23 @@ public class InstanceActor : ReceiveActor
         if (!string.IsNullOrEmpty(request.TriggerAttribute))
         {
             if (!_resolvedAttributeByName.TryGetValue(request.TriggerAttribute, out var tr)
-                || string.IsNullOrEmpty(tr.DataSourceReference)
-                || string.IsNullOrEmpty(tr.BoundDataConnectionName))
+                || string.IsNullOrEmpty(tr.Attribute.DataSourceReference)
+                || string.IsNullOrEmpty(tr.Attribute.BoundDataConnectionName))
             {
                 caller.Tell(new WriteAttributeBatchResponse(
                     cid, false, $"Trigger attribute '{request.TriggerAttribute}' is not data-sourced."));
                 return;
             }
 
-            if (connName != null && !string.Equals(connName, tr.BoundDataConnectionName, StringComparison.Ordinal))
+            if (connName != null && !string.Equals(connName, tr.Attribute.BoundDataConnectionName, StringComparison.Ordinal))
             {
                 caller.Tell(new WriteAttributeBatchResponse(
                     cid, false, "Trigger attribute is on a different data connection."));
                 return;
             }
 
-            connName ??= tr.BoundDataConnectionName;
-            triggerPath = tr.DataSourceReference;
+            connName ??= tr.Attribute.BoundDataConnectionName;
+            triggerPath = tr.Attribute.DataSourceReference;
             triggerVal = request.TriggerEncodedValue;
         }
 
@@ -846,19 +947,19 @@ public class InstanceActor : ReceiveActor
             // (a CLR array/IEnumerable from the SDK) into a typed List<T>. On an
             // element-type mismatch we set the attribute's quality to Bad, log a
             // warning, and skip storing a value rather than crashing the actor.
-            if (resolved != null && IsListAttribute(resolved))
+            if (resolved != null && resolved.IsList)
             {
-                if (TryCoerceListValue(resolved, update.Value, out var typedList))
+                if (TryCoerceListValue(resolved.ElementDataType, update.Value, out var typedList))
                 {
                     HandleAttributeValueChanged(new AttributeValueChanged(
                         _instanceUniqueName, update.TagPath, attrName,
-                        typedList, update.Quality.ToString(), update.Timestamp));
+                        typedList, QualityToString(update.Quality), update.Timestamp));
                 }
                 else
                 {
                     _logger.LogWarning(
                         "List attribute {Instance}.{Attribute} received a value that could not be coerced to List<{Element}>; marking quality Bad",
-                        _instanceUniqueName, attrName, resolved.ElementDataType);
+                        _instanceUniqueName, attrName, resolved.Attribute.ElementDataType);
                     _attributeQualities[attrName] = "Bad";
                     _attributeTimestamps[attrName] = update.Timestamp;
                     var currentValue = _attributes.GetValueOrDefault(attrName);
@@ -883,14 +984,21 @@ public class InstanceActor : ReceiveActor
 
             HandleAttributeValueChanged(new AttributeValueChanged(
                 _instanceUniqueName, update.TagPath, attrName,
-                value, update.Quality.ToString(), update.Timestamp));
+                value, QualityToString(update.Quality), update.Timestamp));
         }
     }
 
-    /// <summary>True if the resolved attribute is declared as a <see cref="DataType.List"/>.</summary>
-    private static bool IsListAttribute(ResolvedAttribute attr) =>
-        Enum.TryParse<DataType>(attr.DataType, ignoreCase: true, out var dt)
-        && dt == DataType.List;
+    /// <summary>Parses a <see cref="ResolvedAttribute.DataType"/> string, defaulting to
+    /// <see cref="DataType.String"/> on an unrecognized value (never throws).</summary>
+    private static DataType ParseDataType(string dataType) =>
+        Enum.TryParse<DataType>(dataType, ignoreCase: true, out var dt) ? dt : DataType.String;
+
+    /// <summary>Parses a <see cref="ResolvedAttribute.ElementDataType"/> string, returning
+    /// <see langword="null"/> when absent or unrecognized (never throws).</summary>
+    private static DataType? ParseElementDataType(string? elementDataType) =>
+        string.IsNullOrEmpty(elementDataType)
+            ? null
+            : (Enum.TryParse<DataType>(elementDataType, ignoreCase: true, out var et) ? et : (DataType?)null);
 
     /// <summary>
     /// Decodes a STATIC (authored / overridden) attribute's canonical value
@@ -903,16 +1011,15 @@ public class InstanceActor : ReceiveActor
     /// bad element, missing element type) degrades to <see langword="null"/> + a
     /// warning — the caller marks the attribute Bad quality. NEVER throws into the
     /// actor.
+    ///
+    /// Takes the already-parsed <paramref name="dataType"/>/<paramref name="elementType"/>
+    /// (perf remediation, arch-review WP1.5) rather than re-parsing from the attribute's
+    /// raw DataType strings — every call site now resolves these once via
+    /// <see cref="AttributeIndexEntry"/> at config-apply time.
     /// </summary>
-    private object? DecodeAttributeValue(ResolvedAttribute attr, string? raw)
+    private object? DecodeAttributeValue(
+        DataType dataType, DataType? elementType, string? raw, string attributeCanonicalName)
     {
-        DataType dataType = Enum.TryParse<DataType>(attr.DataType, ignoreCase: true, out var dt)
-            ? dt
-            : DataType.String;
-        DataType? elementType = string.IsNullOrEmpty(attr.ElementDataType)
-            ? null
-            : (Enum.TryParse<DataType>(attr.ElementDataType, ignoreCase: true, out var et) ? et : null);
-
         try
         {
             return AttributeValueCodec.Decode(raw, dataType, elementType);
@@ -921,30 +1028,28 @@ public class InstanceActor : ReceiveActor
         {
             _logger.LogWarning(ex,
                 "Attribute '{Attr}' on '{Instance}' has an undecodable List value; marking Bad quality",
-                attr.CanonicalName, _instanceUniqueName);
+                attributeCanonicalName, _instanceUniqueName);
             return null; // caller marks quality Bad
         }
     }
 
     /// <summary>
     /// Coerces an incoming data-sourced value (an OPC UA array / IEnumerable)
-    /// into a typed <c>List&lt;elementClrType&gt;</c> matching the attribute's
-    /// <see cref="ResolvedAttribute.ElementDataType"/>. Each element is converted
-    /// with invariant culture (round-trip parse for DateTime). Returns
-    /// <see langword="false"/> on a missing/invalid element type, a non-enumerable
-    /// value, or any element that cannot be coerced — the caller then marks the
-    /// attribute quality Bad. Never throws.
+    /// into a typed <c>List&lt;elementClrType&gt;</c> matching the attribute's element
+    /// type. Each element is converted with invariant culture (round-trip parse for
+    /// DateTime). Returns <see langword="false"/> on a missing/invalid element type, a
+    /// non-enumerable value, or any element that cannot be coerced — the caller then
+    /// marks the attribute quality Bad. Never throws.
+    ///
+    /// Takes the already-parsed <paramref name="elementType"/> (perf remediation,
+    /// arch-review WP1.5) — see <see cref="DecodeAttributeValue"/>.
     /// </summary>
-    private bool TryCoerceListValue(ResolvedAttribute attr, object? incoming, out object? typedList)
+    private bool TryCoerceListValue(DataType? elementType, object? incoming, out object? typedList)
     {
         typedList = null;
 
-        if (string.IsNullOrEmpty(attr.ElementDataType)
-            || !Enum.TryParse<DataType>(attr.ElementDataType, ignoreCase: true, out var elementType)
-            || !AttributeValueCodec.IsValidElementType(elementType))
-        {
+        if (elementType is not { } et || !AttributeValueCodec.IsValidElementType(et))
             return false;
-        }
 
         if (incoming is not System.Collections.IEnumerable enumerable || incoming is string)
             return false;
@@ -956,7 +1061,7 @@ public class InstanceActor : ReceiveActor
             // inside the guarded block means any future change that introduces a
             // throw is caught and turned into a Bad-quality result rather than
             // escaping into the actor and tripping supervision.
-            typedList = AttributeValueCodec.CoerceEnumerable(enumerable, elementType);
+            typedList = AttributeValueCodec.CoerceEnumerable(enumerable, et);
             return true;
         }
         catch (Exception ex)
@@ -964,7 +1069,7 @@ public class InstanceActor : ReceiveActor
             // Any coercion / construction failure → Bad quality, never a crash.
             _logger.LogWarning(ex,
                 "Failed to coerce value to List<{Element}> for instance {Instance}; marking quality Bad",
-                attr.ElementDataType, _instanceUniqueName);
+                et, _instanceUniqueName);
             return false;
         }
     }
@@ -1417,7 +1522,7 @@ public class InstanceActor : ReceiveActor
             // overrides to a typed list (matching the config-default load), set
             // Bad quality on a malformed stored value, and never crash the actor.
             if (_resolvedAttributeByName.TryGetValue(kvp.Key, out var resolved)
-                && IsListAttribute(resolved))
+                && resolved.IsList)
             {
                 // Decode the stored List override (both old array-of-strings
                 // and native-typed forms decode) and re-persist the native form if
@@ -1425,7 +1530,8 @@ public class InstanceActor : ReceiveActor
                 // list and comparing to the stored string detects old-form values
                 // (native → native is byte-identical, so a native value is a no-op).
                 // The re-persist is fire-and-forget and never throws into the actor.
-                var decoded = DecodeAttributeValue(resolved, kvp.Value);
+                var decoded = DecodeAttributeValue(
+                    resolved.DataType, resolved.ElementDataType, kvp.Value, kvp.Key);
                 _attributes[kvp.Key] = decoded;
                 if (decoded is null && !string.IsNullOrEmpty(kvp.Value))
                 {
@@ -1742,4 +1848,11 @@ public class InstanceActor : ReceiveActor
     /// connection whose previous attempt failed or whose response was lost.
     /// </summary>
     private sealed record RetryTagSubscribe(string ConnectionName);
+
+    /// <summary>Self-tell that fires the coalesced static-override persist flush (P4, arch-review WP1.5).</summary>
+    private sealed class FlushStaticOverrides
+    {
+        public static readonly FlushStaticOverrides Instance = new();
+        private FlushStaticOverrides() { }
+    }
 }
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
index 1f97e8c4..63f2c697 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
@@ -345,6 +345,53 @@ public class SiteStorageService
         await command.ExecuteNonQueryAsync();
     }
 
+    /// <summary>
+    /// Batch counterpart of <see cref="SetStaticOverrideAsync"/> (perf remediation,
+    /// arch-review WP1.5): upserts a set of static attribute overrides for one instance
+    /// in a SINGLE connection + transaction with one prepared command re-bound per row.
+    /// Used by the InstanceActor's coalesced flush — mirrors NativeAlarmActor's
+    /// MarkDirtyUpsert/FlushDirtyUpserts (P4) pattern — so a burst of rapid SetAttribute
+    /// calls on an instance collapses into one batched write instead of a write per call.
+    /// </summary>
+    /// <param name="instanceName">The unique name of the instance.</param>
+    /// <param name="overrides">The attribute name/value pairs to upsert (latest value per name).</param>
+    /// <returns>A task that completes when all rows have been committed.</returns>
+    public async Task SetStaticOverridesAsync(
+        string instanceName, IReadOnlyList<(string AttributeName, string Value)> overrides)
+    {
+        if (overrides.Count == 0)
+            return;
+
+        await using var connection = OpenConnection();
+        await using var transaction = (SqliteTransaction)await connection.BeginTransactionAsync();
+
+        await using var command = connection.CreateCommand();
+        command.Transaction = transaction;
+        command.CommandText = @"
+            INSERT INTO static_attribute_overrides (instance_unique_name, attribute_name, override_value, updated_at)
+            VALUES (@name, @attr, @val, @updatedAt)
+            ON CONFLICT(instance_unique_name, attribute_name) DO UPDATE SET
+                override_value = excluded.override_value,
+                updated_at     = excluded.updated_at";
+
+        var pName = command.Parameters.Add("@name", SqliteType.Text);
+        var pAttr = command.Parameters.Add("@attr", SqliteType.Text);
+        var pVal = command.Parameters.Add("@val", SqliteType.Text);
+        var pUpdatedAt = command.Parameters.Add("@updatedAt", SqliteType.Text);
+        pName.Value = instanceName;
+
+        var updatedAt = DateTimeOffset.UtcNow.ToString("O");
+        foreach (var (attributeName, value) in overrides)
+        {
+            pAttr.Value = attributeName;
+            pVal.Value = value;
+            pUpdatedAt.Value = updatedAt;
+            await command.ExecuteNonQueryAsync();
+        }
+
+        await transaction.CommitAsync();
+    }
+
     /// <summary>
     /// Clears all static attribute overrides for an instance.
     /// Called on redeployment to reset overrides.
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Repositories/SiteExternalSystemRepository.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Repositories/SiteExternalSystemRepository.cs
index 1fe373f4..7daba84e 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Repositories/SiteExternalSystemRepository.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Repositories/SiteExternalSystemRepository.cs
@@ -16,6 +16,15 @@ public class SiteExternalSystemRepository : IExternalSystemRepository
 {
     private readonly SiteStorageService _storage;
 
+    /// <summary>
+    /// Shared options for the per-row <c>MethodDefinitionDto</c> JSON parse in
+    /// <see cref="ParseMethodDefinitions"/> (perf remediation, arch-review WP1.5) —
+    /// avoids allocating a new <see cref="JsonSerializerOptions"/> per call. Settings
+    /// preserved exactly; the query shape itself is untouched (WP2.6).
+    /// </summary>
+    private static readonly JsonSerializerOptions MethodDefinitionJsonOptions =
+        new() { PropertyNameCaseInsensitive = true };
+
     /// <summary>
     /// Initializes a new site-side external system repository.
     /// </summary>
@@ -279,7 +288,7 @@ public class SiteExternalSystemRepository : IExternalSystemRepository
         try
         {
             var methods = JsonSerializer.Deserialize<List<MethodDefinitionDto>>(json,
-                new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+                MethodDefinitionJsonOptions);
 
             if (methods is null)
                 return Array.Empty<ExternalSystemMethod>();
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
index 5657c1a8..ee13aea0 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
@@ -242,6 +242,77 @@ public class InstanceActorTests : TestKit, IDisposable
         Assert.Equal("100.0", overrides["Temperature"]);
     }
 
+    /// <summary>
+    /// Perf remediation (arch-review WP1.5): a burst of rapid static-attribute writes must
+    /// coalesce into ONE batched SQLite transaction on a short single-shot timer — mirroring
+    /// NativeAlarmActor's MarkDirtyUpsert/FlushDirtyUpserts (P4) shape — rather than a write
+    /// per <see cref="SetStaticAttributeCommand"/>. Pinned two ways: (1) nothing is persisted
+    /// before the flush interval elapses, even though every write already replied
+    /// synchronously and updated in-memory state; (2) only the LATEST value per attribute
+    /// survives, across writes to two different attributes coalesced into the same flush.
+    /// </summary>
+    [Fact]
+    public async Task InstanceActor_RapidStaticWrites_CoalesceIntoSingleFlush()
+    {
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "PumpCoalesce1",
+            Attributes =
+            [
+                new ResolvedAttribute { CanonicalName = "Counter", Value = "0", DataType = "Int32" },
+                new ResolvedAttribute { CanonicalName = "Label", Value = "Idle", DataType = "String" }
+            ]
+        };
+
+        var flushInterval = TimeSpan.FromSeconds(1);
+        var actor = ActorOf(Props.Create(() => new InstanceActor(
+            "PumpCoalesce1",
+            JsonSerializer.Serialize(config),
+            _storage,
+            _compilationService,
+            _sharedScriptLibrary,
+            null,
+            _options,
+            NullLogger<InstanceActor>.Instance,
+            null,   // dclManager
+            null,   // healthCollector
+            null,   // serviceProvider
+            flushInterval)));
+
+        // Five rapid writes to "Counter" plus one to "Label", all sent back-to-back with no
+        // delay between them (well inside the 1s coalescing window).
+        for (var i = 1; i <= 5; i++)
+        {
+            actor.Tell(new SetStaticAttributeCommand(
+                $"corr-counter-{i}", "PumpCoalesce1", "Counter", i.ToString(), DateTimeOffset.UtcNow));
+        }
+        actor.Tell(new SetStaticAttributeCommand(
+            "corr-label", "PumpCoalesce1", "Label", "Running", DateTimeOffset.UtcNow));
+
+        // Every write replies synchronously (in-memory state is authoritative regardless of
+        // when the buffered persist lands) — six writes, six responses.
+        for (var i = 0; i < 6; i++)
+        {
+            var response = ExpectMsg<SetStaticAttributeResponse>(TimeSpan.FromSeconds(5));
+            Assert.True(response.Success);
+        }
+
+        // Well before the flush interval elapses, NOTHING has reached SQLite yet — proves the
+        // writes did not each fire their own persist (the old per-write fire-and-forget shape
+        // would already show a row here).
+        await Task.Delay(200);
+        var beforeFlush = await _storage.GetStaticOverridesAsync("PumpCoalesce1");
+        Assert.Empty(beforeFlush);
+
+        // After the flush interval elapses, exactly one coalesced write has landed: only the
+        // LATEST value per attribute survives.
+        await Task.Delay(1500);
+        var afterFlush = await _storage.GetStaticOverridesAsync("PumpCoalesce1");
+        Assert.Equal(2, afterFlush.Count);
+        Assert.Equal("5", afterFlush["Counter"]);
+        Assert.Equal("Running", afterFlush["Label"]);
+    }
+
     [Fact]
     public async Task InstanceActor_LoadsStaticOverridesFromSQLite()
     {