diff --git a/packages/signals/tests/attribution-feedback.test.ts b/packages/signals/tests/attribution-feedback.test.ts
index 943526d59..f23e9f4ac 100644
--- a/packages/signals/tests/attribution-feedback.test.ts
+++ b/packages/signals/tests/attribution-feedback.test.ts
@@ -29,6 +29,7 @@ afterEach(() => {
   attribution.disable();
   flush();
   vi.restoreAllMocks();
+  vi.useRealTimers();
 });
 
 const wait = (ms: number) => new Promise(r => setTimeout(r, ms));
@@ -444,6 +445,12 @@ describe("feedback()", () => {
   });
 
   it("measures how long each loading boundary showed its fallback, and counts flashes", async () => {
+    // The show is timed on the engine's clock (`performance.now()`): on the
+    // wall clock a loaded runner stretched the 20ms wait past the 150ms flash
+    // window. Faked, the fallback is up for exactly the time advanced.
+    vi.useFakeTimers({
+      toFake: ["setTimeout", "clearTimeout", "setInterval", "clearInterval", "Date", "performance"]
+    });
     arm();
     const feed = pagedFeed();
     const shown: string[] = [];
@@ -464,12 +471,14 @@ describe("feedback()", () => {
     expect(shown).toEqual(["loading…"]);
     let [row] = feedback().fallbacks;
     expect(row).toMatchObject({ boundary: "boundary", shows: 1, shownMs: 0, flashes: 0 });
-    await wait(20);
+    await vi.advanceTimersByTimeAsync(20);
     feed.resolve("a");
-    await until(() => shown.includes("a-p1"), "content");
+    await vi.advanceTimersByTimeAsync(0);
+    flush();
+    expect(shown).toContain("a-p1");
     [row] = feedback().fallbacks;
     expect(row.shows).toBe(1);
-    expect(row.shownMs).toBeGreaterThanOrEqual(15);
+    expect(row.shownMs).toBe(20);
     expect(row.worstMs).toBe(row.shownMs);
     // Under 150ms: a spinner that flashed.
     expect(row.flashes).toBe(1);
diff --git a/packages/signals/tests/attribution-lean-gate.test.ts b/packages/signals/tests/attribution-lean-gate.test.ts
index 7bb5174ef..773934413 100644
--- a/packages/signals/tests/attribution-lean-gate.test.ts
+++ b/packages/signals/tests/attribution-lean-gate.test.ts
@@ -144,7 +144,11 @@ describe("attribution engine: lean gate", () => {
   it("the checks run without a record: a hot scope still warns", () => {
     const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
     const { setN } = chain();
-    attribution.enable({ log: false, hotRuns: { count: 3, windowMs: 10_000 } });
+    attribution.enable({
+      log: false,
+      hotRuns: { count: 3, windowMs: 10_000 },
+      hotTime: false
+    });
     for (let i = 1; i <= 4; i++) {
       setN(i);
       flush();
diff --git a/packages/signals/tests/observe-idle-cost.test.ts b/packages/signals/tests/observe-idle-cost.test.ts
index e561c7e71..42e1965cd 100644
--- a/packages/signals/tests/observe-idle-cost.test.ts
+++ b/packages/signals/tests/observe-idle-cost.test.ts
@@ -9,97 +9,77 @@
  *
  * Relative tripwire, same discipline as heap-mark-incremental: absolute
  * wall-clock bounds do not survive CI, so the SAME workload runs against the
- * built prod and observe artifacts in one process, interleaved, best-of-k,
- * and the observe/prod ratio is what is capped. Both tiers see the same
- * machine load, and best-of-k picks the quiet run for each.
+ * built prod and observe artifacts in one worker thread, as paired samples,
+ * and the median of the per-pair observe/prod ratios is what is capped. A
+ * pair sees one machine load for both tiers, so contention largely cancels
+ * within it; the median drops the pairs it doesn't.
  */
 import { existsSync } from "node:fs";
 import { dirname, resolve } from "node:path";
-import { fileURLToPath } from "node:url";
+import { fileURLToPath, pathToFileURL } from "node:url";
+import { Worker } from "node:worker_threads";
 import { describe, expect, test } from "vitest";
 
-type Tier = typeof import("../src/index.js");
-
 // The built artifacts — what apps actually resolve. Gitignored, so the test
 // skips when they haven't been built (run `pnpm build`); resolved paths rather
 // than literal specifiers so the build's type pass doesn't try to find them.
 const here = dirname(fileURLToPath(import.meta.url));
 const PROD = resolve(here, "../dist/prod/index.js");
 const OBSERVE = resolve(here, "../dist/observe/index.js");
+const WORKER = resolve(here, "observe-idle-cost.worker.mjs");
 
-/**
- * A graph-heavy workload with no hooks installed: N chains of
- * signal → memo → memo → effect, then K write passes that touch every chain,
- * then teardown. Reads, writes, recomputes, effect runs and creation all
- * cross the observe wiring; nothing observes.
- */
-function workload(tier: Tier, N: number, K: number): number {
-  const { createEffect, createMemo, createRoot, createSignal, flush } = tier;
-  let ms = 0;
-  createRoot(dispose => {
-    const setters: ((v: number) => void)[] = [];
-    let sink = 0;
-    for (let i = 0; i < N; i++) {
-      const [a, setA] = createSignal(i);
-      const b = createMemo(() => a() * 2);
-      const c = createMemo(() => b() + 1);
-      createEffect(
-        () => c(),
-        v => {
-          sink += v;
-        }
-      );
-      setters.push(setA);
-    }
-    flush();
-    const start = performance.now();
-    for (let k = 1; k <= K; k++) {
-      for (let i = 0; i < N; i++) setters[i](i + k);
-      flush();
-    }
-    ms = performance.now() - start;
-    dispose();
-    if (sink === Infinity) throw new Error("unreachable");
+type Samples = {
+  observeDefined: boolean;
+  prodDefined: boolean;
+  prodMs: number[];
+  observeMs: number[];
+};
+
+function measure(): Promise<Samples> {
+  return new Promise((done, fail) => {
+    const worker = new Worker(WORKER, {
+      workerData: {
+        prodUrl: pathToFileURL(PROD).href,
+        observeUrl: pathToFileURL(OBSERVE).href,
+        // ~13ms a sample locally; 2×(warmup + pairs) samples per round.
+        N: 1000,
+        K: 40,
+        warmup: 5,
+        pairs: 21
+      }
+    });
+    worker.once("message", done);
+    worker.once("error", fail);
   });
-  return ms;
 }
 
+const median = (xs: number[]) => [...xs].sort((a, b) => a - b)[xs.length >> 1];
+
 describe.skipIf(!existsSync(PROD) || !existsSync(OBSERVE))("observe tier idle cost", () => {
   test("no hooks installed: the observe artifact runs the same graph within the cap of prod", async () => {
-    const prod = (await import(PROD)) as Tier;
-    const observe = (await import(OBSERVE)) as Tier;
-    expect((prod as any).OBSERVE).toBeUndefined();
-    expect((observe as any).OBSERVE).toBeDefined();
-
-    // ~15ms a sample locally; under a loaded CI worker a full three rounds
-    // stays well inside the explicit timeout below.
-    const N = 1000;
-    const K = 10;
-    // Warm both (JIT, allocator) before anything is timed.
-    workload(prod, N, 2);
-    workload(observe, N, 2);
-    // Measured 2026-09-16 (M-series, five samples): 1.03–1.09 — the wiring
-    // is 3–9% on a graph that does nothing but cross it. The cap trips when
-    // the wiring costs ~3x what it does today (25%), which is the regression
-    // this exists to catch — a hook site that stopped being a null check.
-    // Noise: the suite runs this beside other files on worker
-    // threads, so one tier can draw the busy slots; a round is best-of-k
-    // interleaved, and a round over the cap is re-measured (a regression is
-    // over the cap every round, contention is not).
+    // Measured 2026-10-05 (M-series, Node loading the artifacts directly):
+    // median pair ratio 0.96–1.05. The cap trips when the wiring costs a
+    // quarter of the graph's own work, which is the regression this exists
+    // to catch — a hook site that stopped being a null check. For scale, a
+    // WeakMap bump per write and per recompute reads 1.23–1.28.
+    // Noise: the suite runs this beside other files on worker threads, so
+    // the machine's load shifts under the measurement. Pairing cancels the
+    // shift a pair sees, the median drops the outliers, and a round over
+    // the cap is re-measured (a regression is over the cap every round,
+    // contention is not).
     const CAP = 1.25;
     let best = Infinity;
     let detail = "";
     for (let round = 0; round < 3 && best >= CAP; round++) {
-      let prodMs = Infinity;
-      let observeMs = Infinity;
-      for (let i = 0; i < 5; i++) {
-        prodMs = Math.min(prodMs, workload(prod, N, K));
-        observeMs = Math.min(observeMs, workload(observe, N, K));
-      }
-      const ratio = observeMs / prodMs;
+      const s = await measure();
+      expect(s.prodDefined).toBe(false);
+      expect(s.observeDefined).toBe(true);
+      const ratio = median(s.observeMs.map((o, i) => o / s.prodMs[i]));
       if (ratio < best) {
         best = ratio;
-        detail = `observe ${observeMs.toFixed(1)}ms / prod ${prodMs.toFixed(1)}ms`;
+        detail = `median pair ratio over ${s.prodMs.length} pairs; observe ${median(
+          s.observeMs
+        ).toFixed(1)}ms / prod ${median(s.prodMs).toFixed(1)}ms (medians)`;
       }
     }
     expect(best, detail).toBeLessThan(CAP);
diff --git a/packages/signals/tests/observe-idle-cost.worker.mjs b/packages/signals/tests/observe-idle-cost.worker.mjs
new file mode 100644
index 000000000..3daa0a9ff
--- /dev/null
+++ b/packages/signals/tests/observe-idle-cost.worker.mjs
@@ -0,0 +1,84 @@
+// Measurement half of observe-idle-cost.test.ts. Runs in a worker thread so
+// Node loads the built artifacts itself: inside the test runner they would go
+// through vite's SSR transform, which rewrites every cross-module import of
+// the module-preserving builds into a namespace-object property load — a
+// cost apps never pay, larger than the one under measurement, and not the
+// same for both tiers.
+import { parentPort, workerData } from "node:worker_threads";
+
+const { prodUrl, observeUrl, N, K, warmup, pairs } = workerData;
+const prod = await import(prodUrl);
+const observe = await import(observeUrl);
+
+// This thread's CPU time, in ms: time spent descheduled on a loaded runner
+// is wall-clock noise, not cost. Wall clock where Node predates the API.
+const clock = process.threadCpuUsage
+  ? () => {
+      const { user, system } = process.threadCpuUsage();
+      return (user + system) / 1000;
+    }
+  : () => performance.now();
+
+/**
+ * A graph-heavy workload with no hooks installed: N chains of
+ * signal → memo → memo → effect, then K write passes that touch every chain,
+ * then teardown. Reads, writes, recomputes and effect runs cross the observe
+ * wiring in the timed window; nothing observes.
+ */
+function workload(tier) {
+  const { createEffect, createMemo, createRoot, createSignal, flush } = tier;
+  let ms = 0;
+  createRoot(dispose => {
+    const setters = [];
+    let sink = 0;
+    for (let i = 0; i < N; i++) {
+      const [a, setA] = createSignal(i);
+      const b = createMemo(() => a() * 2);
+      const c = createMemo(() => b() + 1);
+      createEffect(
+        () => c(),
+        v => {
+          sink += v;
+        }
+      );
+      setters.push(setA);
+    }
+    flush();
+    const start = clock();
+    for (let k = 1; k <= K; k++) {
+      for (let i = 0; i < N; i++) setters[i](i + k);
+      flush();
+    }
+    ms = clock() - start;
+    dispose();
+    if (sink === Infinity) throw new Error("unreachable");
+  });
+  return ms;
+}
+
+// Paired samples: each pair runs the tiers back to back, alternating which
+// goes first, so both halves of a pair see the same machine load and neither
+// tier owns the slot after (say) a GC of the other's garbage.
+function pair(i) {
+  if (i % 2) {
+    const o = workload(observe);
+    return [workload(prod), o];
+  }
+  const p = workload(prod);
+  return [p, workload(observe)];
+}
+
+for (let i = 0; i < warmup; i++) pair(i);
+const prodMs = [];
+const observeMs = [];
+for (let i = 0; i < pairs; i++) {
+  const [p, o] = pair(i);
+  prodMs.push(p);
+  observeMs.push(o);
+}
+parentPort.postMessage({
+  observeDefined: observe.OBSERVE !== undefined,
+  prodDefined: prod.OBSERVE !== undefined,
+  prodMs,
+  observeMs
+});
diff --git a/packages/web/test/loading.spec.tsx b/packages/web/test/loading.spec.tsx
index 09c8cf844..dc7d3ddab 100644
--- a/packages/web/test/loading.spec.tsx
+++ b/packages/web/test/loading.spec.tsx
@@ -892,10 +892,7 @@ describe("Testing Loading", () => {
 // root insert until the fetch landed: the page stayed blank — not even
 // content OUTSIDE the boundary mounted — and the fallback never showed.
 describe("<Loading> around an optimistic store's first flight", () => {
-  // Real timers: the fetches below settle on their own schedule.
-  beforeEach(() => {
-    vi.useRealTimers();
-  });
+  const FETCH_MS = 10;
   const wait = (ms: number) => new Promise<void>(r => setTimeout(r, ms));
 
   type Item = { id: number; label: string; votes: number };
@@ -905,7 +902,7 @@ describe("<Loading> around an optimistic store's first flight", () => {
     const [list] = make<Item[]>(async () => {
       tick();
       const n = ++fetches;
-      await wait(10);
+      await wait(FETCH_MS);
       return [{ id: 1, label: "Tacos", votes: n - 1 }];
     }, [] as Item[]);
     return { list, setTick };
@@ -952,17 +949,17 @@ describe("<Loading> around an optimistic store's first flight", () => {
     const { html, dispose, refetch } = mount(createOptimisticStore);
     expect(html()).toBe("<h1>Poll</h1><div>Loading...</div>");
 
-    await wait(30);
+    await vi.advanceTimersByTimeAsync(FETCH_MS);
     flush();
     expect(html()).toBe("<h1>Poll</h1><ul><li>Tacos - 0</li></ul>");
 
     // Refetch: stale-while-revalidate — no fallback, then the new truth.
     refetch();
     flush();
-    await wait(2);
+    await vi.advanceTimersByTimeAsync(FETCH_MS - 1);
     flush();
     expect(html()).toBe("<h1>Poll</h1><ul><li>Tacos - 0</li></ul>");
-    await wait(30);
+    await vi.advanceTimersByTimeAsync(1);
     flush();
     expect(html()).toBe("<h1>Poll</h1><ul><li>Tacos - 1</li></ul>");
     dispose();
@@ -971,7 +968,7 @@ describe("<Loading> around an optimistic store's first flight", () => {
   test("control: createStore(fn, seed) in the same spot", async () => {
     const { html, dispose } = mount(createStore);
     expect(html()).toBe("<h1>Poll</h1><div>Loading...</div>");
-    await wait(30);
+    await vi.advanceTimersByTimeAsync(FETCH_MS);
     flush();
     expect(html()).toBe("<h1>Poll</h1><ul><li>Tacos - 0</li></ul>");
     dispose();