Performance Optimization

Overview

Measure before optimizing. Performance work without measurement is guessing — and guessing leads to premature optimization that adds complexity without improving what matters. Profile first, identify the actual bottleneck, fix it, measure again. Optimize only what measurements prove matters.

When to Use

• Performance requirements exist in the spec (load time budgets, response time SLAs)
• Users or monitoring report slow behavior
• Core Web Vitals scores are below thresholds
• You suspect a change introduced a regression
• Building features that handle large datasets or high traffic

When NOT to use: Don't optimize before you have evidence of a problem. Premature optimization adds complexity that costs more than the performance it gains.

Core Web Vitals Targets

| Metric | Good | Needs Improvement | Poor | |--------|------|-------------------|------| | LCP (Largest Contentful Paint) | ≤ 2.5s | ≤ 4.0s | > 4.0s | | INP (Interaction to Next Paint) | ≤ 200ms | ≤ 500ms | > 500ms | | CLS (Cumulative Layout Shift) | ≤ 0.1 | ≤ 0.25 | > 0.25 |

The Optimization Workflow

1. MEASURE  → Establish baseline with real data
2. IDENTIFY → Find the actual bottleneck (not assumed)
3. FIX      → Address the specific bottleneck
4. VERIFY   → Measure again; keep or revert
5. GUARD    → Add monitoring or tests to prevent regression

Step 1: Measure

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');

Where to Start Measuring

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

Step 2: Identify the Bottleneck

Common bottlenecks by category:

Frontend:

| Symptom | Likely Cause | Investigation | |---------|-------------|---------------| | Slow LCP | Large images, render-blocking resources, slow server | Check network waterfall, image sizes | | High CLS | Images without dimensions, late-loading content, font shifts | Check layout shift attribution | | Poor INP | Heavy JavaScript on main thread, large DOM updates | Check long tasks in Performance trace | | Slow initial load | Large bundle, many network requests | Check bundle size, code splitting |

Backend:

| Symptom | Likely Cause | Investigation | |---------|-------------|---------------| | Slow API responses | N+1 queries, missing indexes, unoptimized queries | Check database query log | | Memory growth | Leaked references, unbounded caches, large payloads | Heap snapshot analysis | | CPU spikes | Synchronous heavy computation, regex backtracking | CPU profiling | | High latency | Missing caching, redundant computation, network hops | Trace requests through the stack |

Step 3: Fix Common Anti-Patterns

N+1 Queries (Backend)

// BAD: N+1 — one query per task for the owner
const tasks = await db.tasks.findMany();
for (const task of tasks) {
  task.owner = await db.users.findUnique({ where: { id: task.ownerId } });
}

// GOOD: Single query with join/include
const tasks = await db.tasks.findMany({
  include: { owner: true },
});

Unbounded Data Fetching

// BAD: Fetching all records
const allTasks = await db.tasks.findMany();

// GOOD: Paginated with limits
const tasks = await db.tasks.findMany({
  take: 20,
  skip: (page - 1) * 20,
  orderBy: { createdAt: 'desc' },
});

Queries That Ignore Their Index

"Add an index" is the guess. The query plan is the measurement:

EXPLAIN ANALYZE
SELECT id, title FROM tasks
WHERE owner_id = 42 ORDER BY created_at DESC LIMIT 20;

Three things in the output decide the fix:

| What you see | What it means | |---|---| | Seq Scan on a large table where you expected an index | No usable index for this predicate | | Estimated rows= off from actual by an order of magnitude | Stale statistics; the planner is choosing on bad information | | A Sort node above the scan | The index covers the filter but not the ORDER BY |

Index for the shape of the query, not the column in isolation. In a composite index, equality columns come first, then the range or sort column:

CREATE INDEX idx_tasks_owner_created ON tasks (owner_id, created_at DESC);

When an index will not help:

| Situation | Why | |---|---| | Low selectivity, querying the dominant value (a status column that is 95% active, filtered on active) | A sequential scan is genuinely cheaper; the planner will ignore the index. Filtering on the rare value is the opposite case, and a partial index serves it well | | Leading wildcard (LIKE '%term') | A B-tree cannot seek without a prefix; needs trigram or full-text | | Function on the column (WHERE lower(email) = ?) | The plain column index is unusable; index the expression instead | | Write-heavy table | Every index is a tax on every INSERT/UPDATE; measure the write cost, not just the read gain |

Re-run EXPLAIN ANALYZE after. An index that did not change the plan is a revert (Step 4), and it is not free: it still costs on every write.

Connection Pool Exhaustion

The signature is distinctive: every endpoint slows at once, the slow time is spent waiting for a connection rather than executing, and the database reports mostly idle sessions.

// BAD: a pool per request or per module — under serverless this multiplies
// by instance count and exhausts the database's connection limit
// GOOD: one pool per process, sized against the database's ceiling
const pool = new Pool({
  max: 10,                        // instances × max must stay under max_connections
  idleTimeoutMillis: 30_000,
  connectionTimeoutMillis: 5_000, // fail fast instead of queueing forever
});

Bigger is not faster. A pool larger than what the database can execute concurrently just relocates the queue from your app to the database, where it is harder to see. When instance count is unbounded (serverless, autoscaling), a proxy that multiplexes connections (pgbouncer, RDS Proxy) is the fix, not a higher max.

Missing Image Optimization (Frontend)

<!-- BAD: No dimensions, no format optimization -->
<img src="/hero.jpg" />

<!-- GOOD: Hero / LCP image — art direction + resolution switching, high priority -->
<!--
  Two techniques combined:
  - Art direction (media): different crop/composition per breakpoint
  - Resolution switching (srcset + sizes): right file size per screen density
-->
<picture>
  <!-- Mobile: portrait crop (8:10) -->
  <source
    media="(max-width: 767px)"
    srcset="/hero-mobile-400.avif 400w, /hero-mobile-800.avif 800w"
    sizes="100vw"
    width="800"
    height="1000"
    type="image/avif"
  />
  <source
    media="(max-width: 767px)"
    srcset="/hero-mobile-400.webp 400w, /hero-mobile-800.webp 800w"
    sizes="100vw"
    width="800"
    height="1000"
    type="image/webp"
  />
  <!-- Desktop: landscape crop (2:1) -->
  <source
    srcset="/hero-800.avif 800w, /hero-1200.avif 1200w, /hero-1600.avif 1600w"
    sizes="(max-width: 1200px) 100vw, 1200px"
    width="1200"
    height="600"
    type="image/avif"
  />
  <source
    srcset="/hero-800.webp 800w, /hero-1200.webp 1200w, /hero-1600.webp 1600w"
    sizes="(max-width: 1200px) 100vw, 1200px"
    width="1200"
    height="600"
    type="image/webp"
  />
  <img
    src="/hero-desktop.jpg"
    width="1200"
    height="600"
    fetchpriority="high"
    alt="Hero image description"
  />
</picture>

<!-- GOOD: Below-the-fold image — lazy loaded + async decoding -->
<img
  src="/content.webp"
  width="800"
  height="400"
  loading="lazy"
  decoding="async"
  alt="Content image description"
/>

Unnecessary Re-renders (React)

// BAD: Creates new object on every render, causing children to re-render
function TaskList() {
  return <TaskFilters options={{ sortBy: 'date', order: 'desc' }} />;
}

// GOOD: Stable reference
const DEFAULT_OPTIONS = { sortBy: 'date', order: 'desc' } as const;
function TaskList() {
  return <TaskFilters options={DEFAULT_OPTIONS} />;
}

// Use React.memo for expensive components
const TaskItem = React.memo(function TaskItem({ task }: Props) {
  return <div>{/* expensive render */}</div>;
});

// Use useMemo for expensive computations
function TaskStats({ tasks }: Props) {
  const stats = useMemo(() => calculateStats(tasks), [tasks]);
  return <div>{stats.completed} / {stats.total}</div>;
}

Large Bundle Size

// Modern bundlers (Vite, webpack 5+) handle named imports with tree-shaking automatically,
// provided the dependency ships ESM and is marked `sideEffects: false` in package.json.
// Profile before changing import styles — the real gains come from splitting and lazy loading.

// GOOD: Dynamic import for heavy, rarely-used features
const ChartLibrary = lazy(() => import('./ChartLibrary'));

// GOOD: Route-level code splitting wrapped in Suspense
const SettingsPage = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <Suspense fallback={<Spinner />}>
      <SettingsPage />
    </Suspense>
  );
}

Missing Caching (Backend)

Cache what is expensive to produce and read far more often than it changes. Caching a query that was already fast adds a network hop, a staleness bug, and an eviction policy to maintain, in exchange for nothing.

Pick the layer deliberately:

| Layer | Visible to | Use when | Cost | |---|---|---|---| | In-process (Map, LRU) | One instance | Small, hot, per-instance staleness is acceptable | Each instance drifts independently; invalidation reaches only one | | Shared (Redis, Memcached) | All instances | Instances must agree, or the value is expensive to recompute | A network hop, and another service to run and monitor | | CDN / edge | Everyone, per URL | Responses are public and identical for a given key | Invalidation is the hard part; assume you cannot recall a bad response quickly |

// Cache frequently-read, rarely-changed data
const CACHE_TTL = 5 * 60 * 1000; // 5 minutes
let cachedConfig: AppConfig | null = null;
let cacheExpiry = 0;

async function getAppConfig(): Promise<AppConfig> {
  if (cachedConfig && Date.now() < cacheExpiry) {
    return cachedConfig;
  }
  cachedConfig = await db.config.findFirst();
  cacheExpiry = Date.now() + CACHE_TTL;
  return cachedConfig;
}

// HTTP caching headers for static assets
app.use('/static', express.static('public', {
  maxAge: '1y',           // Cache for 1 year
  immutable: true,        // Never revalidate (use content hashing in filenames)
}));

// Cache-Control for API responses
res.set('Cache-Control', 'public, max-age=300'); // 5 minutes

Key design decides correctness. 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, and that ships as a performance win.

Choose one invalidation strategy, not three:

| Strategy | Trade-off | |---|---| | TTL | Simplest. You accept staleness up to the TTL, so state the acceptable window explicitly | | Event or tag based | Fresh on write, but writers now have to know the cache topology | | Versioned keys (user:42:profile:v7) | Never invalidate, just stop reading old keys. Costs memory until eviction |

Guard against the stampede. A hot key expires, every concurrent request misses together, and the origin takes the full load at once, which is how a cache turns into an outage instead of preventing one. Serve stale while a single request recomputes (stale-while-revalidate), or coalesce concurrent misses behind one in-flight promise so N waiters cause one recompute.

Do not cache: anything whose staleness is a correctness bug (balances, permissions, inventory at checkout), or per-user data under a key that does not identify the user. See ../../references/performance-checklist.md for request coalescing, write strategies, negative caching, and the cache checklist.

Step 4: Verify (Keep or Revert)

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:

| Result vs. baseline | Action | |---|---| | Past the threshold, tests green | Keep. Commit with the before/after numbers in the message. | | Within noise (no measurable change) | Revert. | | Worse | Revert. | | Improved, but a test went red | Revert. A regression wearing a win's clothing. |

"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.

Log every attempt, including the reverted ones

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:

| Idea | Baseline → Result | Verdict | Why | |---|---|---|---| | Memoize the row component | INP 240ms → 235ms | reverted | Inside noise (±15ms). Rows weren't the bottleneck. | | Virtualize the list | INP 240ms → 90ms | kept | Long tasks gone from the trace. | | Preconnect to the API origin | LCP 2.8s → 2.8s | reverted | Already same-origin. |

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.

Step 5: Guard Against Regression

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

See Also

For detailed performance checklists, optimization commands, and anti-pattern reference, see ../../references/performance-checklist.md.

Common Rationalizations

| Rationalization | Reality | |---|---| | "We'll optimize later" | Performance debt compounds. Fix obvious anti-patterns now, defer micro-optimizations. | | "It's fast on my machine" | Your machine isn't the user's. Profile on representative hardware and networks. | | "This optimization is obvious" | If you didn't measure, you don't know. Profile first. | | "Users won't notice 100ms" | Research shows 100ms delays impact conversion rates. Users notice more than you think. | | "The framework handles performance" | Frameworks prevent some issues but can't fix N+1 queries or oversized bundles. | | "The query is slow, add an index" | Read the plan first. The index may already exist and be unusable, and every index taxes writes forever. | | "Just cache it" | Caching an already-cheap call buys nothing and adds a staleness bug. Cache what is expensive and re-read far more than written. | | "Raise the pool size, we're running out of connections" | A pool bigger than the database can serve moves the queue somewhere less visible. Find what holds connections. | | "It didn't help much, but it doesn't hurt" | Neutral changes are a revert. You pay maintenance on them forever and got nothing back. | | "We already wrote it, may as well keep it" | Sunk cost. The measurement doesn't care how long the change took to write. | | "The improvement is obvious, no need to re-measure" | Then re-measuring is cheap and proves it. Unmeasured wins are how neutral complexity lands. |

Red Flags

• Optimization without profiling data to justify it
• N+1 query patterns in data fetching
• An index added without a query plan before and after to justify it
• A cache key that omits an input the response depends on (tenant, locale, viewer)
• A cache with no stated staleness window and no invalidation strategy
• Connection pool size raised in response to exhaustion, without finding what holds connections
• List endpoints without pagination
• Images without dimensions, lazy loading, or responsive sizes
• Bundle size growing without review
• No performance monitoring in production
• React.memo and useMemo everywhere (overusing is as bad as underusing)
• Optimizations kept without a re-measurement that justifies them
• Several optimizations bundled into one measurement, so no single change can be attributed
• A "win" that required a test to be changed, skipped, or deleted
• The same failed optimization attempted more than once because nobody recorded the first attempt

Verification

After any performance-related change:

• [ ] Before and after measurements exist (specific numbers)
• [ ] The result was re-measured the same way as the baseline (same command, same conditions)
• [ ] The improvement exceeds run-to-run variance, not just the mean
• [ ] Changes that didn't beat the baseline were reverted, not kept as neutral
• [ ] Attempts are logged, kept and reverted alike, so a dead idea isn't re-run
• [ ] The specific bottleneck is identified and addressed
• [ ] Core Web Vitals are within "Good" thresholds
• [ ] Bundle size hasn't increased significantly
• [ ] No N+1 queries in new data fetching code
• [ ] Any new index is justified by a query plan before and after, and its write cost was considered
• [ ] Any new cache states what it keys on and how it goes stale
• [ ] The measured user-facing metric has a synthetic budget or field monitor that can detect regression
• [ ] Existing tests still pass (optimization didn't break behavior)