fix(core): Speed up Redux DevTools serialization of timestamps

.changeset/devtools-date-format-perf.md

@@ -0,0 +1,12 @@
+---
+'@data-client/core': patch
+'@data-client/react': patch
+'@data-client/vue': patch
+---
+
+Fix slow updates in development while Redux DevTools is open
+
+With the [Redux DevTools](https://dataclient.io/docs/getting-started/debugging) extension open, every store update
+stalled the page while DevTools serialized the store: about 20ms with 50 entities, and 200ms with 500. Apps doing
+many `controller.set()` calls, polling, or live updates would stutter in development. Each update now serializes
+40-60x faster, and timestamps still show as readable times like `10:42:07.123 AM`.

packages/core/src/manager/DevtoolsManager.ts

@@ -32,7 +32,15 @@ if (process.env.NODE_ENV !== 'production') {
     return serial;
   }
 
-  const HASINTL = typeof Intl !== 'undefined';
+  const dateFormat =
+    typeof Intl !== 'undefined' ?
+      new Intl.DateTimeFormat('en-US', {
+        hour: 'numeric',
+        minute: 'numeric',
+        second: 'numeric',
+        fractionalSecondDigits: 3,
+      })
+    : undefined;
   DEFAULT_CONFIG = {
     name: `Data Client: ${globalThis.document?.title}`,
     autoPause: true,
@@ -59,20 +67,15 @@ if (process.env.NODE_ENV !== 'production') {
       options: undefined,
       /* istanbul ignore next */
       replacer:
-        HASINTL ?
+        dateFormat ?
           (key: string | number | symbol, value: unknown) => {
             if (
               typeof value === 'number' &&
               typeof key === 'string' &&
               isFinite(value) &&
               (key === 'date' || key.endsWith('At'))
             ) {
-              return Intl.DateTimeFormat('en-US', {
-                hour: 'numeric',
-                minute: 'numeric',
-                second: 'numeric',
-                fractionalSecondDigits: 3,
-              }).format(value);
+              return dateFormat.format(value);
             }
             return value;
           }

website/blog/2026-10-03-v0.19-batch-set.md

@@ -34,6 +34,7 @@ endpoint code about 2x faster, and fixes a round of TypeScript and Vue issues.
 - [Entity](/rest/api/Entity) classes can be used where an `EntityInterface` is expected ([#4149](https://github.com/reactive/data-client/pull/4149)); [prepare `pk()` overrides](/blog/2026/10/03/v0.19-batch-set#entity-pk-args) for a future release
 - [useCache()](/docs/api/useCache) and [useDLE()](/docs/api/useDLE) return `undefined` for a deleted entity whose refetch failed, instead of a truthy `Symbol` that slipped past `if (!data)` checks; [remove any workarounds](/blog/2026/10/03/v0.19-batch-set#deleted-entity-undefined) ([#4150](https://github.com/reactive/data-client/pull/4150))
 - Vue [useFetch()](/vue/api/useFetch) keeps its data from being garbage collected while mounted, like [useSuspense()](/vue/api/useSuspense), so a configured `gcPolicy` no longer evicts prefetched data that components read later ([#4152](https://github.com/reactive/data-client/pull/4152))
+- Apps with [Redux DevTools](/docs/getting-started/debugging) open no longer stutter in development on large stores or frequent updates; each update [serializes 40-60x faster](/blog/2026/10/03/v0.19-batch-set#devtools-perf) ([#4163](https://github.com/reactive/data-client/pull/4163))
 - Vue [useFetch()](/vue/api/useFetch) is typed as the read-only `Ref` it returns, so `promise.resolved` (always `undefined`) is now a TypeScript error; read `promise.value.resolved` instead ([#4114](https://github.com/reactive/data-client/pull/4114))
 
 **[Breaking Changes:](/blog/2026/10/03/v0.19-batch-set#migration-guide)**
@@ -492,6 +493,25 @@ function TodoDetail({ id }: { id: number }) {
 To show something specific when the refetch failed (for example a 404 after deletion), read `error` from
 [useDLE()](/docs/api/useDLE) rather than inspecting `data`.
 
+### Faster Redux DevTools {#devtools-perf}
+
+With the [Redux DevTools](/docs/getting-started/debugging) extension open, every store update in development
+serializes the whole store for the extension, and each timestamp in it was formatted the slow way. Large stores or
+frequent updates, like polling, live data or many `controller.set()` calls, made the page stutter. Each update now
+serializes **40-60x faster**, and timestamps still read like `10:42:07.123 AM`
+([#4163](https://github.com/reactive/data-client/pull/4163)). Production builds don't include DevTools, so they are
+unaffected.
+
+<PerfChart
+  title="DevTools serialization per store update"
+  baselineLabel="v0.18"
+  valueLabel="v0.19"
+  rows={[
+    { label: '50 entities', baseline: 18.6, value: 0.31 },
+    { label: '500 entities', baseline: 201, value: 5.0 },
+  ]}
+/>
+
 ## Migration guide
 
 import PkgTabs from '@site/src/components/PkgTabs';