diff --git a/packages/html/vitest.config.ts b/packages/html/vitest.config.ts
index 6fc9357e..d7720366 100644
--- a/packages/html/vitest.config.ts
+++ b/packages/html/vitest.config.ts
@@ -8,5 +8,13 @@ export default defineConfig({
     passWithNoTests: true,
     onConsoleLog: (log) => !log.includes('Lit is in dev mode'),
     environment: 'happy-dom',
+    // Some define/* tests dynamically `await import()` composite modules
+    // that pull a large graph through Vite's transform pipeline. Under
+    // `pnpm test` at the workspace root (turbo runs every package's
+    // vitest in parallel, plus Chromium for spf/dom), CPU contention
+    // can push a single import past Vitest's 5s default. Raise it so the
+    // tests stay reliable under load without masking a real regression —
+    // anything approaching this ceiling would still fail.
+    testTimeout: 15_000,
   },
 });