Performance Guard Conventions

Apply when a performance-guard check fails in CI, or when touching a route or endpoint flagged by the live bun scripts/perf-hotspots.ts report.

Overview

Stella guards performance the same way it guards schema safety: committed baselines, diffed on every run. A regression either fails CI outright or shows up as a reviewable diff in the PR. Five guards exist today:

• Network baseline (apps/web/e2e/network-baseline.json, checked by apps/web/e2e/helpers/network.ts from apps/web/e2e/specs/route-smoke.spec.ts): per-route request manifest, waterfall depth, per-request repeat budget, and per-request DB query budget.
• Bundle baseline (scripts/bundle-baseline.ts + scripts/bundle-baseline.json): gzipped size per vendor/entry/route chunk group, wired into the web-build CI job via --check.
• React Compiler bailout guard (scripts/rc-bailouts.ts + scripts/react-compiler-bailouts.json): tracks every component the compiler cannot memoize, so a bailout losing its manual useMemo/useCallback fails CI instead of silently reintroducing an infinite-update-loop risk.
• DB query counter (apps/api/src/lib/db-query-counter.ts): the runtime half of the network baseline's DB-query budget. Dev/test only.
• require-loader-prefetch oxlint rule (.oxlint-plugins/require-loader-prefetch.ts): static, not baseline-based; flags the waterfall pattern the network baseline would otherwise only catch after the fact.

The Core Norm

Fix the regression first. A red guard means the change made something slower, heavier, or chattier than before. Reseeding the baseline to make CI green is not a mechanical step; it is a product decision that the regression is acceptable, and it must be justified in the PR description (why the extra request, the deeper wait, or the bigger chunk is worth it).

The network baseline has two write modes for exactly this distinction:

• E2E_NETWORK_BASELINE=write merges into the existing baseline: requests accumulate as a union, depth and DB-query budgets take the max. Safe to run repeatedly (e.g. to re-accumulate timing-conditional requests); used for legitimate additions such as a new endpoint a route now legitimately calls.
• E2E_NETWORK_BASELINE=rewrite snapshots from scratch, discarding anything not observed on this run. Use it after a perf fix, to tighten a depth or budget back down. Follow a rewrite with a few write runs to re-accumulate any timing-conditional requests the single rewrite run missed.

The bundle baseline mirrors this with --write-baseline (regenerate) and a RATCHET_DOWN prompt (not a failure) when a chunk shrinks by more than 3%, so a real win gets locked in rather than silently drifting back up. The RC bailout guard and the query counter follow the same "commit the smaller number, don't just silence the check" norm.

Failure Playbook

New request on route

network.ts reports New API request(s) on <route>. The route now calls an endpoint it did not before. If intentional, write the baseline. If not, find what changed (a new hook mount, a widened select, an added useQuery) and remove the call.

Request waterfall got deeper

Request waterfall got deeper on <route>: N -> M. Each extra level is one more sequential network round the user waits through. The fix is almost always to start the query in the route loader instead of the component: prefetch it with ensureRouteQueryData (blocking, critical data) or prefetchRouteQuery (non-blocking warmup) from apps/web/src/lib/react-query.ts, so the fetch starts during navigation in parallel with code-split chunk loading, and the component's useSuspenseQuery consumes an already-warm cache instead of opening a new round. require-loader-prefetch catches the same pattern statically before it ever reaches the baseline: it flags useSuspenseQuery(factory(...)) when the route has no loader, or has one that never references factory.

Request repeated more than budgeted

API request repeated on <route>: <key> ran N -> M times. Baseline budgets per-request-key repeat counts (default 1 unless the committed requestCounts says otherwise). Duplicate firing usually comes from duplicate component mounts, normalized UUID fan-out (multiple ids hitting the same :id-normalized key), or a refetch policy that lets the same endpoint fire twice. Reuse the in-flight query instead of issuing a second one.

DB query count grew

DB queries per request grew on <route>: <key> ran N -> M queries. The classic cause is an N+1: a per-row query inside a loop, or a lazy relation loaded once per item instead of preloaded. Batch it (joins, IN lists, Drizzle relation preloading); see /conventions-db for indexing and batching patterns. The allowance (dbQueryAllowance, budget + max(2, 15%)) already absorbs normal noise (auth session-refresh piggybacks, cache variance); a failure here is a real regression, not jitter.

If instead the check reports DB query count missing on <route>, the response stopped exposing the dev/test x-db-queries header — restore the query counter wiring before trusting the route's N+1 budget again.

Bundle group over budget

The bundle baseline fails when a named group (entry, a vendor-* chunk, or routes/largest-route) exceeds its committed gzip size by more than 3% (or 1 KiB, whichever is larger, per HEADROOM/HEADROOM_FLOOR_BYTES). Two specific failure shapes:

• A dependency escaped its manualChunks bucket and landed in entry (paid on every cold visit) instead of a lazy route chunk or vendor-* group. Dynamic-import() it, or fix the manualChunks rule in apps/web/vite.config.ts.
• vendor-anonymize-data or wasm-vendor show up nonzero. These are tracked at 0 because they should only ever load inside a web worker, never the main client bundle. A nonzero value means a worker-only dependency leaked into the client graph; keep it worker-only instead of widening the baseline.

require-loader-prefetch lint failure

Same underlying problem as "waterfall got deeper," caught statically instead of at e2e time: a route component calls useSuspenseQuery(factory(...)) but the route's loader either doesn't exist or never references factory. Prefetch factory(...) in the loader via ensureRouteQueryData or prefetchRouteQuery.

How Depth Is Measured

waterfallDepth (apps/web/e2e/helpers/network.ts) counts the most consecutive busy blocks in any one observation sequence: a busy block is a maximal run of requests whose intervals overlap, and a launch gap over REQUEST_SEQUENCE_GAP_MS (500) starts a new sequence so an idle prefetch is not read as another route-load round. Two requests in flight at the same instant are never two levels, so the number is a lower bound on true causal depth: a dependent request hiding behind an unrelated slow one is invisible to it.

Reading launch times rather than the gap since the previous response ended is what buys monotonicity, and it costs sensitivity: a dependent request whose parent ran longer than 500ms opens a new sequence instead of counting a level. The three properties are not simultaneously satisfiable, because separating a dependent request from an idle prefetch requires the parent's duration, and any rule reading response ends lets a response growing under load close a gap it used to exceed. Under-counting is the safer error for a guard compared only upward.

The metric is load-monotone: neither a slower response nor a uniformly stretched timeline can raise it. Sequence boundaries read launch times only, so a longer response cannot move one; coverage is a running max that is never rewound at a boundary, so a split can only lower the count.

One residual remains, and it is what the +1 DEPTH_JITTER_ALLOWANCE in assertNetworkBaseline covers: two requests issued from different ticks may overlap on one run and not the next, shifting a route by exactly one block. No launch-time metric can tell that apart from a real added round. The allowance is capped at one level and must stay there, because the metric can no longer carry a count across an observation boundary: a route reporting two extra levels has genuinely grown one, so investigate the request graph rather than re-running. Only write/rewrite when the route's behavior actually changed.

Live Hotspot Burn-Down

Do not embed a dated hotspot snapshot in instructions; it becomes false while remaining authoritative-looking. Before touching a hot route or endpoint, run:

bun scripts/perf-hotspots.ts

Treat the reported budgets as recorded debt, not acceptable targets. Capture the relevant before value, fix or avoid worsening the access path, then run the same command and affected guard again. When a real improvement lowers a baseline, commit the tighter value. When an intentional product capability raises one, document the measured tradeoff in the PR rather than hiding it in a generic baseline refresh.

For database changes, pair the route/query counter with the actual query plan and cardinality. A lower request count can still conceal a slower scan, and a fast development database does not validate a production-size access path.

Cross-Links

• /conventions-scale — pagination and tenant-scoped queries; a query that ignores these will also blow the DB-query budget.
• /conventions-db — indexes, batching, relation preloading; the concrete fix for most N+1 failures above.
• /conventions-ux — GPU-friendly animation and skeleton conventions; a waterfall fix that adds a loading state should use a real structural skeleton, not a spinner.