Load route chunks in parallel with beforeLoad

[dpkass]

Route chunks start loading only after every beforeLoad in the match has resolved (executeClientLane runs contextualize before planSuccessfulLane, whose loader tasks call loadRouteChunk).

That's fine when beforeLoad is fast. Ours isn't: an ssr: false authenticated layout initializes Keycloak and fetches the user's organization in beforeLoad, ~500 ms at 120 ms latency. The layout and page chunks sit idle during that time.

Start used to hide this, because the HTML had modulepreload links for those chunks. After updating Start 1.168.34 → 1.169.4, chunks below the first ssr: false match are no longer preloaded. ~80 modules now start only after beforeLoad. Cold-cache time to home went from 1,339 to 1,433 ms.

Workaround we use:

router.subscribe("onBeforeLoad", ({ toLocation }) => {
  const [routes] = router.getMatchedRoutes(toLocation.pathname);
  for (const route of routes) void router.loadRouteChunk(route);
});

Cold-cache time to home: 1,203 ms. Warm cache unaffected.

Proposal: an option to start loading matched route chunks in parallel with beforeLoad, or Start keeping the modulepreload links for ssr: false routes. If waiting is deliberate (don't download code for a page a guard rejects), an opt-in keeps that default.

Happy to open a PR.

Versions: @tanstack/react-router 1.170.46, @tanstack/router-core 1.171.39, @tanstack/react-start 1.169.4.

Drafted with AI assistance; the measurements are from our app.