What

Adds a per-scan memo for the raw fs.readdirSync(dir, { withFileTypes: true }) calls made while scanning the App Router graph, via a small readDirEntriesCached helper backed by a WeakMap<ValidFileMatcher, Map<string, Dirent[]>> — the same per-scan-matcher pattern already used by findFileProbeCache and findSlotSubPagesCache. discoverParallelSlots and findSlotRootPage now read through it.

Why

discoverInheritedParallelSlots walks every ancestor directory of each route and readdirSyncs it to look for @slot directories. Because routes share ancestors, a directory with N sibling routes is read once per descendant route — the scan is super-linear (≈ O(routes × ancestor-dir-size)) in the width of a route directory. findFileProbeCache already memoizes findFile probes, but not these raw readdirSync/existsSync calls.

The App Router graph is scanned both during production build and at dev-server startup, so this affects both.

Measured

Stress fixture with a wide/flat route directory (worst case for this pathology), Apple M-series:

Production build (vinext build)

Dev server cold start (launch → root route serving, .vite cache cleared)

The route scan runs at every dev-server (re)start, independent of the optimizeDeps/.vite cache — so the route-scan portion of this win applies to warm restarts too, not only cleared-cache cold starts. Output is byte-identical (29,578 module-transforms with and without the cache). The win scales super-linearly with route-directory width: negligible for a handful of routes, dramatic for apps with wide route directories.

Memory safety

The cache is a WeakMap keyed on the per-scan matcher clone (scanMatcher, created fresh in buildAppRouteGraph), identical in lifetime to the existing findFileProbeCache / findSlotSubPagesCache. Its entries are released as soon as the scan's matcher is garbage-collected, so nothing accumulates across rebuilds/restarts in a long-lived dev server — a module-level Map would leak here, but the WeakMap-per-scan-matcher does not. Within a single scan the retained set is bounded by the app's directory count, and it replaces the much larger transient Dirent churn the un-cached repeated reads produced.

Correctness

• readDirEntriesCached returns [] for missing/unreadable directories, preserving the prior existsSync-guard and try/catch semantics of discoverParallelSlots / findSlotRootPage.
• Route/slot suites pass — app-route-graph, slot, app-optimistic-routing, app-rsc-route-matching, hybrid-route-priority, app-page-route-wiring, pages-router: 564 tests.
• vp lint clean.

(The 10k stress fixture was generated with the opt-in VINEXT_BENCH_EXTRA_ROUTES knob from #2388.)