React Performance

Comprehensive React performance guidance covering measurement, rendering optimization, bundle size, Core Web Vitals, and Server Component patterns. Verified against React docs and Web Vitals specifications.

First principle: Measure before you optimize. This skill teaches you to find problems before fixing them.

Architecture Overview

[Diagnose]
    ├── React DevTools Profiler (component render times)
    ├── Chrome Performance Panel (main thread activity)
    ├── web-vitals library (LCP, INP, CLS in production)
    └── Bundle analyzer (what's in the bundle)
              ↓
[Identify Root Cause]
    ├── Unnecessary re-renders? → Memoization / state colocation
    ├── Slow initial load? → Code splitting / Server Components
    ├── Janky interactions? → useTransition / useDeferredValue
    ├── Layout shifts? → Image/font optimization
    └── Large bundle? → Dynamic imports / tree shaking
              ↓
[Fix — Least Invasive First]
    ├── 1. Restructure components (children pattern, local state)
    ├── 2. Enable React Compiler (auto-memoization)
    ├── 3. Use concurrent features (transitions, deferred values)
    ├── 4. Move work to Server Components
    ├── 5. Code split heavy dependencies
    └── 6. Manual memo/useMemo/useCallback (last resort)

Quick Reference

Need to...	See
Measure performance before optimizing	Profiling and Measurement
Fix unnecessary re-renders	Rendering Optimization
Reduce JavaScript bundle size	Bundle Optimization
Fix LCP, INP, or CLS scores	Core Web Vitals
Decide Server vs Client Components	Server Components
Handle large lists and data sets	Data and Lists
Optimize images, fonts, and media	Assets
Decide when React Compiler handles it	React Compiler

Decision Matrix: Which Optimization?

"My page loads slowly"

Symptom	Likely Cause	Fix	Resource
LCP > 2.5s, large image	Unoptimized hero image	next/image with priority	assets
LCP > 2.5s, text content	Render-blocking JS	Server Components, code split	server-components, bundle
LCP > 2.5s, font flash	Font loading delay	next/font	assets
TTFB > 800ms	Slow data fetching	Parallel fetches, streaming	server-components
Large JS bundle (> 200KB)	Too much client code	Dynamic imports, tree shaking	bundle

"My page feels janky/unresponsive"

Symptom	Likely Cause	Fix	Resource
INP > 200ms on typing	Heavy re-renders on input	useTransition, state colocation	rendering
INP > 200ms on click	Expensive event handler	startTransition, requestIdleCallback	rendering
INP > 200ms on navigation	Route transition re-renders	useTransition for navigation	rendering
Scrolling stutter	Large list rendering	Virtualization	data-and-lists
Input lag	Context re-rendering entire tree	Split context, selector pattern	rendering

"Layout is jumping around"

Symptom	Likely Cause	Fix	Resource
CLS > 0.1, images	Missing width/height	next/image or explicit dimensions	assets, web-vitals
CLS > 0.1, fonts	Font swap	next/font	assets
CLS > 0.1, dynamic content	Content inserted after load	Reserve space, Suspense fallback	web-vitals
CLS > 0.1, animations	Layout-triggering CSS	Use transform instead of top/left	web-vitals

"Should I use memo/useMemo/useCallback?"

Is React Compiler enabled?
├── YES → Don't add manual memoization. Compiler handles it.
└── NO
    ├── Have you measured with Profiler?
    │   ├── NO → Measure first. The problem might not be re-renders.
    │   └── YES, renders are slow (> 1ms per component)
    │       ├── Can you restructure? (children pattern, local state)
    │       │   ├── YES → Do that instead. No memo needed.
    │       │   └── NO → Use memo/useMemo/useCallback.
    │       └── Is the child already wrapped in memo()?
    │           ├── NO → Wrapping parent's callback in useCallback alone does nothing.
    │           └── YES → useCallback/useMemo will help.
    └── Can you enable React Compiler?
        ├── YES → Enable it. Remove manual memoization later.
        └── NO → Use memo/useMemo/useCallback where Profiler shows benefit.

The Optimization Hierarchy

The React team's official priority order:

1. Write correct code first — fix bugs, don't mask them with memoization
2. Structure components well — local state, children pattern, avoid unnecessary Effects
3. Measure before optimizing — use Profiler, console.time, CPU throttling in production builds
4. Adopt React Compiler — handles memo/useMemo/useCallback automatically
5. Use concurrent features (useTransition, useDeferredValue, Suspense) for responsiveness
6. Use Server Components to eliminate client bundle weight and fetch waterfalls
7. Manual memo/useMemo/useCallback only as escape hatches when the compiler is insufficient

Anti-Patterns Checklist

When reviewing code, watch for these performance killers:

• Effect chains — useEffect that sets state triggering another useEffect (derive during render instead)
• Inline object/function props to memo()-wrapped children (defeats memoization)
• State too high — input state at page level re-rendering entire tree
• useEffect for derived state — setFiltered(items.filter(...)) in effect (use useMemo or calculate during render)
• Missing key prop — or using array index as key for dynamic lists
• Context monolith — one Provider with user + theme + cart + notifications
• Client Component boundary too high — 'use client' at layout/page level instead of leaf components
• Heavy imports in client bundle — syntax highlighters, markdown parsers, date libraries not code-split
• Images without dimensions — causes CLS
• onPaste={e => e.preventDefault()} on password fields — blocks password managers (also an a11y violation)
• Manual memo/useMemo/useCallback without measuring first — adds complexity for no proven benefit
• new ExpensiveThing() in useRef initializer — runs every render (use lazy init pattern)