## Summary
Under high request load the webapp spends most of its CPU inside
react-router's `matchRoutes`, not in application code.
`@remix-run/router@1.23.2` (the React Router v6 / Remix 2 core)
re-flattens, re-ranks, and recompiles the entire route table on every
request, and with the webapp's ~436 routes that cost dominates once
request rates climb. There is no `NODE_ENV` gate, so production pays it
too.
This adds a pnpm patch that memoizes the parts that depend only on the
static route manifest: it caches the flattened/ranked branches per route
tree, hoists the loop-invariant `decodePath` out of the match loop, and
caches compiled path regexes.
## Benchmark
CPU profile over the same load (100 concurrent tag feeds, ~425 req/s),
`NODE_ENV=production`, before vs after the patch:
| Metric | Before | After |
| --- | --- | --- |
| Active CPU (self-time over the window) | 28.3s | 18.5s (-34%) |
| Route-matching self-time | 19.2s | 7.5s (-61%) |
| Event-loop lag p99 | 322ms | 113ms (-65%) |
| Idle headroom | 26% | 52% |
Application/realtime code was ~0% of CPU in both profiles; the
bottleneck was entirely generic per-request route matching.
## Why a patch instead of an upgrade
The inefficiency is acknowledged upstream
([remix-run/react-router#8653](https://github.com/remix-run/react-router/issues/8653)).
A contributor PR doing exactly this
([remix-run/react-router#14866](https://github.com/remix-run/react-router/pull/14866))
was closed in favor of a narrower fix
([remix-run/react-router#14967](https://github.com/remix-run/react-router/pull/14967),
branch caching only, shipped in React Router v7), with the maintainer
suggesting patch-package as the interim until the Remix 3 route-pattern
rewrite (see
[remix-run/remix#4786](https://github.com/remix-run/remix/discussions/4786)).
We are on the v6-era core and cannot pick up even the partial fix
without a framework migration, so this patch is the sanctioned stopgap,
and it also includes the compiled-regex cache the merged PR left out.
[`patches/README.md`](https://github.com/triggerdotdev/trigger.dev/blob/perf/react-router-route-matching/patches/README.md)
documents the full rationale, the safety argument (deterministic,
internal-only, bounded caches), and when to remove the patch.
Eric Allam