performance-optimization

Two complementary approaches — use both:

• Synthetic (Lighthouse, DevTools Performance tab): Controlled conditions, reproducible. Best for CI regression detection and isolating specific issues.
• RUM (web-vitals library, CrUX): Real user data in real conditions. Required to validate that a fix actually improved user experience.

Frontend:

# Synthetic: Lighthouse in Chrome DevTools (or CI)
# Chrome DevTools → Performance tab → Record
# Chrome DevTools MCP → Performance trace

# RUM: Web Vitals library in code
import { onLCP, onINP, onCLS } from 'web-vitals';

onLCP(console.log);
onINP(console.log);
onCLS(console.log);

Backend:

# Response time logging
# Application Performance Monitoring (APM)
# Database query logging with timing

# Simple timing
console.time('db-query');
const result = await db.query(...);
console.timeEnd('db-query');

Use the symptom to decide what to measure first:

What is slow?
├── First page load
│   ├── Large bundle? --> Measure bundle size, check code splitting
│   ├── Slow server response? --> Measure TTFB in DevTools Network waterfall
│   │   ├── DNS long? --> Add dns-prefetch / preconnect for known origins
│   │   ├── TCP/TLS long? --> Enable HTTP/2, check edge deployment, keep-alive
│   │   └── Waiting (server) long? --> Profile backend, check queries and caching
│   └── Render-blocking resources? --> Check network waterfall for CSS/JS blocking
├── Interaction feels sluggish
│   ├── UI freezes on click? --> Profile main thread, look for long tasks (>50ms)
│   ├── Form input lag? --> Check re-renders, controlled component overhead
│   └── Animation jank? --> Check layout thrashing, forced reflows
├── Page after navigation
│   ├── Data loading? --> Measure API response times, check for waterfalls
│   └── Client rendering? --> Profile component render time, check for N+1 fetches
└── Backend / API
    ├── Single endpoint slow? --> Profile database queries, check indexes
    ├── All endpoints slow? --> Check connection pool, memory, CPU
    └── Intermittent slowness? --> Check for lock contention, GC pauses, external deps

Common bottlenecks by category:

Frontend:

Backend:

Fix the one thing Step 2 identified, nothing else. The anti-patterns below are the usual suspects; each entry gives the rule and the signature to recognize it, and links to a worked fix in references/optimization-patterns.md. Open the one you need when you reach that code, not before.

Backend

• N+1 queries. One query per row is the most common backend bottleneck. Fetch the relation in the same query (join/include) instead of in the loop. Pattern.
• Unbounded data fetching. Every list endpoint paginates with a limit and a stable order. Pattern.
• Queries that ignore their index. "Add an index" is the guess; EXPLAIN ANALYZE is the measurement. A Seq Scan where you expected an index, a rows= estimate off by an order of magnitude, and a Sort node above the scan each call for a different fix; a bad rows= estimate means stale statistics, so run ANALYZE rather than adding an index. Index for the shape of the query (equality columns first, then the range or sort column). A plain index will not help a query on a low-selectivity dominant value (a partial index serves the rare value), a leading wildcard (needs trigram or full-text), or a function applied to the column (index the expression, as in WHERE lower(email) = ?), and every index taxes every write. Re-run the plan afterwards; an index that did not change it is a revert. Pattern.
• Connection pool exhaustion. Signature: every endpoint slows at once, time is spent waiting for a connection rather than executing, and the database shows mostly idle sessions. One pool per process, sized so instances × max stays under the database's connection ceiling. Bigger is not faster; it relocates the queue to the database where it is harder to see. With unbounded instance counts (serverless, autoscaling), multiplex through a proxy (pgbouncer, RDS Proxy) instead of raising max. Pattern.
• Missing caching. Cache what is expensive to produce and read far more often than it changes; caching an already-fast query adds a network hop and a staleness bug in exchange for nothing. Pick the layer deliberately (in-process, shared, CDN). Every input that changes the response belongs in the key (tenant, locale, permissions, feature flags): a key that omits the viewer is how one user's data gets served to another. Choose one invalidation strategy (TTL, event or tag based, versioned keys) and state the acceptable staleness window explicitly. Guard hot keys against the stampede: serve stale while one request recomputes, or coalesce concurrent misses behind a single in-flight promise. Never cache what must be fresh (balances, permissions, inventory at checkout). Pattern; request coalescing, write strategies, and negative caching in ../../references/performance-checklist.md.

Frontend

• Missing image optimization. Every image declares width and height (CLS). The LCP image gets fetchpriority="high", modern formats (AVIF, WebP) through <picture>, and srcset/sizes for resolution switching; below-the-fold images get loading="lazy" and decoding="async". Pattern.
• Unnecessary re-renders. An object or function created in render is a new reference every time and re-renders every child that receives it. Hoist constants; reserve React.memo and useMemo for work the profile shows is expensive, since overuse is its own cost. Pattern.
• Large bundle size. Modern bundlers tree-shake ESM named imports on their own; the real gains are route-level code splitting and lazy-loading heavy, rarely-used features behind Suspense. Profile before changing import styles. Pattern.

A fix is a hypothesis until you re-measure. This step decides whether it survives.

Re-measure the way you measured the baseline: same command, same conditions, same fixed budget (wall-clock, sample count, or request count). A baseline taken on a cold cache against a result taken on a warm one measures the cache, not your change.

Change one thing at a time. Three optimizations landed together produce one number, and you cannot attribute it. If they must ship together, measure each in isolation first.

Beat the noise, not just the mean. Repeat the measurement and compare the delta against run-to-run variance. A 3% gain inside ±5% variance is not a gain; it is a different sample.

Then decide, strictly:

"Neutral" is a revert, not a keep. This is the step teams skip: the change is already written, throwing it away feels wasteful, so it lands unmeasured, and the codebase accretes complexity that never bought anything. Code you keep, you maintain forever. Make it pay for itself.

Correctness gates the metric. The suite stays green and the number moves. An "optimization" that wins by dropping work the product needed (skipping a validation, caching something that must be fresh, removing an await that was load-bearing) is a regression, not a win.

Reverted work leaves no trace in git history, which is exactly why the same dead idea gets tried again next quarter. Keep a short ledger so a discarded idea stays discarded:

A section in the PR description or a PERF.md in the repo both work. What matters is that the next person (or the next agent) reads it before proposing an experiment, and doesn't re-run one that already failed.

Guard the metric the user actually feels, not every available number. Use the same LCP, INP, p95 latency, or other primary metric that justified the fix.

Use two complementary layers when the surface is user-facing:

• Synthetic CI gate: Catch reproducible regressions before merge with a performance budget. Repeat noisy measurements or compare a median/trend so normal run-to-run variance does not turn the gate into a flaky check.
• Field monitoring: Alert on a meaningful p75 movement in RUM data. Use attributed web-vitals data to locate the cause; treat CrUX's rolling window as confirmation rather than an immediate alert.

When either guard fires, return to Step 1 and establish a fresh baseline before proposing another fix.

Set budgets and enforce them:

JavaScript bundle: < 200KB gzipped (initial load)
CSS: < 50KB gzipped
Images: < 200KB per image (above the fold)
Fonts: < 100KB total
API response time: < 200ms (p95)
Time to Interactive: < 3.5s on 4G
Lighthouse Performance score: ≥ 90

Enforce in CI:

# Bundle size check
npx bundlesize --config bundlesize.config.json

# Lighthouse CI
npx lhci autorun