diff --git a/package.json b/package.json
index a8ea927a4..b157c1d23 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,7 @@
     "docs:api": "bun sdk/generate-api-docs.ts",
     "docs:api:check": "bun sdk/generate-api-docs.ts --check",
     "fetch-collision-data": "bun sdk/fetch-collision-data.ts",
-    "test": "bun test sdk/test server/webclient/src/bot",
+    "test": "bun test sdk/test server/webclient/src/bot server/webclient/src/util",
     "typecheck": "tsc --noEmit",
     "typecheck:webclient": "tsc --noEmit -p server/webclient/tsconfig.json"
   },
diff --git a/server/webclient/src/util/JsUtil.ts b/server/webclient/src/util/JsUtil.ts
index 2bc9aa570..e88e86b90 100644
--- a/server/webclient/src/util/JsUtil.ts
+++ b/server/webclient/src/util/JsUtil.ts
@@ -1,4 +1,8 @@
-export const sleep = async (ms: number): Promise<void> => new Promise((resolve): NodeJS.Timeout => setTimeout(resolve, ms));
+import { delay } from '#/util/WorkerTimer.js';
+
+// Backed by a worker clock so the main loop keeps its rate in a background tab,
+// where Chrome would otherwise throttle setTimeout to one wake per second.
+export const sleep = async (ms: number): Promise<void> => delay(ms);
 
 export const downloadUrl = async (url: string): Promise<Uint8Array> => new Uint8Array(await (await fetch(url)).arrayBuffer());
 
diff --git a/server/webclient/src/util/WorkerTimer.test.ts b/server/webclient/src/util/WorkerTimer.test.ts
new file mode 100644
index 000000000..eaed4ddd9
--- /dev/null
+++ b/server/webclient/src/util/WorkerTimer.test.ts
@@ -0,0 +1,124 @@
+import { describe, expect, test } from 'bun:test';
+
+type WorkerTimerModule = typeof import('./WorkerTimer.js');
+
+/**
+ * Load a fresh copy of the module. It memoises whether a worker is available,
+ * so each scenario needs its own instance.
+ */
+let variant = 0;
+async function freshModule(): Promise<WorkerTimerModule> {
+    return import(`./WorkerTimer.js?variant=${variant++}`) as Promise<WorkerTimerModule>;
+}
+
+/** A worker that echoes each id back after its delay, like the real one. */
+class EchoWorker {
+    onmessage: ((event: { data: number }) => void) | null = null;
+    onerror: (() => void) | null = null;
+    posted: { id: number; ms: number }[] = [];
+
+    postMessage(data: { id: number; ms: number }): void {
+        this.posted.push(data);
+        setTimeout(() => this.onmessage?.({ data: data.id }), data.ms);
+    }
+
+    terminate(): void {}
+}
+
+/** Install stubs for the globals the module builds a worker from. */
+function withWorkerGlobals(factory: () => unknown): () => void {
+    const saved = {
+        Worker: (globalThis as Record<string, unknown>).Worker,
+        Blob: (globalThis as Record<string, unknown>).Blob,
+        URL: (globalThis as Record<string, unknown>).URL
+    };
+    const realUrl = globalThis.URL;
+
+    (globalThis as Record<string, unknown>).Worker = factory;
+    (globalThis as Record<string, unknown>).Blob = class {};
+    (globalThis as Record<string, unknown>).URL = Object.assign(
+        function URLStub() {} as unknown as typeof realUrl,
+        { createObjectURL: () => 'blob:stub', revokeObjectURL: () => {} }
+    );
+
+    return () => {
+        (globalThis as Record<string, unknown>).Worker = saved.Worker;
+        (globalThis as Record<string, unknown>).Blob = saved.Blob;
+        (globalThis as Record<string, unknown>).URL = saved.URL;
+    };
+}
+
+describe('WorkerTimer', () => {
+    test('falls back to setTimeout when workers are unavailable', async () => {
+        const saved = (globalThis as Record<string, unknown>).Worker;
+        delete (globalThis as Record<string, unknown>).Worker;
+
+        try {
+            const { delay, isWorkerTimerActive } = await freshModule();
+
+            expect(isWorkerTimerActive()).toBe(false);
+
+            const start = performance.now();
+            await delay(20);
+            expect(performance.now() - start).toBeGreaterThanOrEqual(15);
+        } finally {
+            (globalThis as Record<string, unknown>).Worker = saved;
+        }
+    });
+
+    test('runs on a real worker when the platform provides one', async () => {
+        const { delay, isWorkerTimerActive } = await freshModule();
+
+        expect(isWorkerTimerActive()).toBe(true);
+
+        const start = performance.now();
+        await delay(20);
+        expect(performance.now() - start).toBeGreaterThanOrEqual(15);
+    });
+
+    test('drives delays through the worker when one can be created', async () => {
+        let created: EchoWorker | null = null;
+        const restore = withWorkerGlobals(function (this: unknown) {
+            created = new EchoWorker();
+            return created;
+        });
+
+        try {
+            const { delay, isWorkerTimerActive } = await freshModule();
+
+            expect(isWorkerTimerActive()).toBe(true);
+            await delay(5);
+            await delay(7);
+
+            expect(created!.posted.map(p => p.ms)).toEqual([5, 7]);
+            // Ids must be distinct, or one reply would resolve the wrong waiter.
+            expect(new Set(created!.posted.map(p => p.id)).size).toBe(2);
+        } finally {
+            restore();
+        }
+    });
+
+    test('releases waiters and stops using a worker that errors', async () => {
+        let created: EchoWorker | null = null;
+        const restore = withWorkerGlobals(function (this: unknown) {
+            created = new EchoWorker();
+            return created;
+        });
+
+        try {
+            const { delay, isWorkerTimerActive } = await freshModule();
+
+            // A delay long enough that only the error can resolve it in time.
+            const waiting = delay(60_000);
+            created!.onerror?.();
+
+            await waiting; // hangs forever if the error path drops its waiters
+            expect(isWorkerTimerActive()).toBe(false);
+
+            // Later calls still work, now on setTimeout.
+            await delay(1);
+        } finally {
+            restore();
+        }
+    });
+});
diff --git a/server/webclient/src/util/WorkerTimer.ts b/server/webclient/src/util/WorkerTimer.ts
new file mode 100644
index 000000000..2dbaecc3e
--- /dev/null
+++ b/server/webclient/src/util/WorkerTimer.ts
@@ -0,0 +1,113 @@
+// A clock that keeps running while the tab is in the background.
+//
+// Chrome clamps a hidden page's setTimeout to one wake per second, and after
+// about five minutes of intensive throttling to one per minute. GameShell's
+// main loop is a sleep() loop, so a backgrounded client stops draining the
+// packet buffer and falls minutes behind: a bot left running in an unfocused
+// tab goes stale and its scripts report "Stuck" while nothing moves.
+//
+// Timers inside a dedicated worker are exempt from that throttling, so the
+// worker owns the clock and posts back once each delay elapses. Measured in
+// Chrome 146 with the tab hidden: 1.0 page wake/s against 66.4 worker wakes/s,
+// where the foreground rate was 66.0.
+//
+// Falls back to setTimeout wherever a worker can't be created — no Worker or
+// Blob constructor, a Content-Security-Policy that forbids blob: workers, or
+// the worker failing at runtime.
+
+const WORKER_SOURCE = 'self.onmessage=e=>{setTimeout(()=>self.postMessage(e.data.id),e.data.ms)};';
+
+/** undefined = not tried yet, null = unavailable, Worker = in use. */
+let worker: Worker | null | undefined;
+let blobUrl: string | null = null;
+let nextId: number = 1;
+
+const pending: Map<number, () => void> = new Map();
+
+/** Stop using the worker and let every waiter through, so nothing hangs. */
+function abandonWorker(): void {
+    worker = null;
+    const waiters: (() => void)[] = [...pending.values()];
+    pending.clear();
+    for (const resolve of waiters) {
+        resolve();
+    }
+}
+
+function getWorker(): Worker | null {
+    if (worker !== undefined) {
+        return worker;
+    }
+
+    worker = null;
+    try {
+        if (typeof Worker === 'undefined' || typeof Blob === 'undefined' || typeof URL === 'undefined' || typeof URL.createObjectURL !== 'function') {
+            return worker;
+        }
+
+        blobUrl = URL.createObjectURL(new Blob([WORKER_SOURCE], { type: 'text/javascript' }));
+        const created: Worker = new Worker(blobUrl);
+
+        created.onmessage = (event: MessageEvent): void => {
+            // The first reply proves the worker script loaded, so the blob URL
+            // has served its purpose and can be released.
+            if (blobUrl) {
+                URL.revokeObjectURL(blobUrl);
+                blobUrl = null;
+            }
+
+            const id: number = event.data as number;
+            const resolve: (() => void) | undefined = pending.get(id);
+            if (resolve) {
+                pending.delete(id);
+                resolve();
+            }
+        };
+
+        created.onerror = (): void => {
+            created.terminate();
+            if (blobUrl) {
+                URL.revokeObjectURL(blobUrl);
+                blobUrl = null;
+            }
+            abandonWorker();
+        };
+
+        worker = created;
+    } catch {
+        worker = null;
+    }
+
+    return worker;
+}
+
+/**
+ * Resolve after `ms` milliseconds, at the requested rate even when the tab is
+ * in the background. Drop-in replacement for a setTimeout-backed sleep.
+ */
+export async function delay(ms: number): Promise<void> {
+    const timer: Worker | null = getWorker();
+    if (!timer) {
+        return new Promise<void>((resolve): void => {
+            setTimeout(resolve, ms);
+        });
+    }
+
+    return new Promise<void>((resolve): void => {
+        const id: number = nextId++;
+        pending.set(id, resolve);
+        try {
+            timer.postMessage({ id, ms });
+        } catch {
+            // The worker died between the check and the post.
+            pending.delete(id);
+            abandonWorker();
+            setTimeout(resolve, ms);
+        }
+    });
+}
+
+/** True while the worker clock is driving delays. Exposed for diagnostics. */
+export function isWorkerTimerActive(): boolean {
+    return getWorker() !== null;
+}