diff --git a/demo/benchmark_large_house.mjs b/demo/benchmark_large_house.mjs
index ed64754b..499ee584 100644
--- a/demo/benchmark_large_house.mjs
+++ b/demo/benchmark_large_house.mjs
@@ -5,6 +5,7 @@ import { dirname, resolve } from 'node:path';
 import { launch } from './serve.mjs';
 import { LARGE_HOUSE_COUNTS, makeLargeHouseFixture } from './fixtures/large-house.mjs';
 import { assertFreshDemoBundle } from './bundle-freshness.mjs';
+import { ensureHarnessEditorRuntime } from './editor-runtime-compat.mjs';
 import { summarizeLongTasks, summarizeTimings } from './performance/evaluate.mjs';
 import { assertCardContract, LARGE_HOUSE_CARD_CONTRACT } from './performance/card-contract.mjs';
 
@@ -52,6 +53,9 @@ await page.addStyleTag({
 await page.addScriptTag({
   content: `window.__hpAssertCardContract = ${assertCardContract.toString()};`,
 });
+await page.addScriptTag({
+  content: `window.__hpEnsureHarnessEditorRuntime = ${ensureHarnessEditorRuntime.toString()};`,
+});
 const chromium = await browser.version();
 let buildFingerprint;
 try {
@@ -187,6 +191,11 @@ try {
         card._onLabsSnapshot({ active: Object.freeze(['iso']), space: '' });
       }
       card.hass = hassFor(fixture.states);
+      // launch() warms its own bootstrap card, but each measured sample creates
+      // a fresh card. Current lazy bundles therefore need their own controller
+      // construction; monolithic stable bundles remain immediate (#380).
+      if (!await window.__hpEnsureHarnessEditorRuntime(card))
+        throw new Error('large-house editor runtime did not preload');
       window.__hpAssertCardContract(card, cardContract);
       if (requiresIsometric && (typeof card._setProjection !== 'function'
           || !(card._isoGeometryCache instanceof Map))) {
diff --git a/test/performance-contract.test.mjs b/test/performance-contract.test.mjs
index 4f0f6634..c1b19a6b 100644
--- a/test/performance-contract.test.mjs
+++ b/test/performance-contract.test.mjs
@@ -37,6 +37,16 @@ test('large-house benchmark declares every private card member it consumes', ()
   );
 });
 
+test('every measured large-house card preloads its own lazy editor runtime (#380)', () => {
+  const source = readFileSync(
+    new URL('../demo/benchmark_large_house.mjs', import.meta.url), 'utf8',
+  );
+  const create = source.indexOf("document.createElement('houseplan-card')");
+  const preload = source.indexOf('await window.__hpEnsureHarnessEditorRuntime(card)', create);
+  const contract = source.indexOf('window.__hpAssertCardContract(card, cardContract)', create);
+  assert.ok(create >= 0 && preload > create && contract > preload);
+});
+
 test('Glow benchmark declares every private card member it consumes', () => {
   const declared = new Set([
     ...declaredMembers(GLOW_CARD_CONTRACT),