Identify and fix performance issues to create faster, smoother user experiences.

Assess Performance Issues

Understand current performance and identify problems:

Measure current state:

• Core Web Vitals: LCP, FID/INP, CLS scores

• Load time: Time to interactive, first contentful paint

• Bundle size: JavaScript, CSS, image sizes

• Runtime performance: Frame rate, memory usage, CPU usage

• Network: Request count, payload sizes, waterfall

Identify bottlenecks:

• What's slow? (Initial load? Interactions? Animations?)

• What's causing it? (Large images? Expensive JavaScript? Layout thrashing?)

• How bad is it? (Perceivable? Annoying? Blocking?)

• Who's affected? (All users? Mobile only? Slow connections?)

CRITICAL: Measure before and after. Premature optimization wastes time. Optimize what actually matters.

Optimization Strategy

Create systematic improvement plan:

Loading Performance

Optimize Images:

• Use modern formats (WebP, AVIF)

• Proper sizing (don't load 3000px image for 300px display)

• Lazy loading for below-fold images

• Responsive images (srcset , picture element)

• Compress images (80-85% quality is usually imperceptible)

• Use CDN for faster delivery

<img src="hero.webp" srcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w" sizes="(max-width: 400px) 400px, (max-width: 800px) 800px, 1200px" loading="lazy" alt="Hero image" />

Reduce JavaScript Bundle:

• Code splitting (route-based, component-based)

• Tree shaking (remove unused code)

• Remove unused dependencies

• Lazy load non-critical code

• Use dynamic imports for large components

// Lazy load heavy component const HeavyChart = lazy(() => import('./HeavyChart'));

Optimize CSS:

• Remove unused CSS

• Critical CSS inline, rest async

• Minimize CSS files

• Use CSS containment for independent regions

Optimize Fonts:

• Use font-display: swap or optional

• Subset fonts (only characters you need)

• Preload critical fonts

• Use system fonts when appropriate

• Limit font weights loaded

@font-face { font-family: 'CustomFont'; src: url('/fonts/custom.woff2') format('woff2'); font-display: swap; /* Show fallback immediately / unicode-range: U+0020-007F; / Basic Latin only */ }

Optimize Loading Strategy:

• Critical resources first (async/defer non-critical)

• Preload critical assets

• Prefetch likely next pages

• Service worker for offline/caching

• HTTP/2 or HTTP/3 for multiplexing

Rendering Performance

Avoid Layout Thrashing:

// ❌ Bad: Alternating reads and writes (causes reflows) elements.forEach(el => { const height = el.offsetHeight; // Read (forces layout) el.style.height = height * 2; // Write });

// ✅ Good: Batch reads, then batch writes const heights = elements.map(el => el.offsetHeight); // All reads elements.forEach((el, i) => { el.style.height = heights[i] * 2; // All writes });

Optimize Rendering:

• Use CSS contain property for independent regions

• Minimize DOM depth (flatter is faster)

• Reduce DOM size (fewer elements)

• Use content-visibility: auto for long lists

• Virtual scrolling for very long lists (react-window, react-virtualized)

Reduce Paint & Composite:

• Use transform and opacity for animations (GPU-accelerated)

• Avoid animating layout properties (width, height, top, left)

• Use will-change sparingly for known expensive operations

• Minimize paint areas (smaller is faster)

Animation Performance

GPU Acceleration:

/* ✅ GPU-accelerated (fast) */ .animated { transform: translateX(100px); opacity: 0.5; }

/* ❌ CPU-bound (slow) */ .animated { left: 100px; width: 300px; }

Smooth 60fps:

• Target 16ms per frame (60fps)

• Use requestAnimationFrame for JS animations

• Debounce/throttle scroll handlers

• Use CSS animations when possible

• Avoid long-running JavaScript during animations

Intersection Observer:

// Efficiently detect when elements enter viewport const observer = new IntersectionObserver((entries) => { entries.forEach(entry => { if (entry.isIntersecting) { // Element is visible, lazy load or animate } }); });

React/Framework Optimization

React-specific:

• Use memo() for expensive components

• useMemo() and useCallback() for expensive computations

• Virtualize long lists

• Code split routes

• Avoid inline function creation in render

• Use React DevTools Profiler

Framework-agnostic:

• Minimize re-renders

• Debounce expensive operations

• Memoize computed values

• Lazy load routes and components

Network Optimization

Reduce Requests:

• Combine small files

• Use SVG sprites for icons

• Inline small critical assets

• Remove unused third-party scripts

Optimize APIs:

• Use pagination (don't load everything)

• GraphQL to request only needed fields

• Response compression (gzip, brotli)

• HTTP caching headers

• CDN for static assets

Optimize for Slow Connections:

• Adaptive loading based on connection (navigator.connection)

• Optimistic UI updates

• Request prioritization

• Progressive enhancement

Core Web Vitals Optimization

Largest Contentful Paint (LCP < 2.5s)

• Optimize hero images

• Inline critical CSS

• Preload key resources

• Use CDN

• Server-side rendering

First Input Delay (FID < 100ms) / INP (< 200ms)

• Break up long tasks

• Defer non-critical JavaScript

• Use web workers for heavy computation

• Reduce JavaScript execution time

Cumulative Layout Shift (CLS < 0.1)

• Set dimensions on images and videos

• Don't inject content above existing content

• Use aspect-ratio CSS property

• Reserve space for ads/embeds

• Avoid animations that cause layout shifts

/* Reserve space for image */ .image-container { aspect-ratio: 16 / 9; }

Performance Monitoring

Tools to use:

• Chrome DevTools (Lighthouse, Performance panel)

• WebPageTest

• Core Web Vitals (Chrome UX Report)

• Bundle analyzers (webpack-bundle-analyzer)

• Performance monitoring (Sentry, DataDog, New Relic)

Key metrics:

• LCP, FID/INP, CLS (Core Web Vitals)

• Time to Interactive (TTI)

• First Contentful Paint (FCP)

• Total Blocking Time (TBT)

• Bundle size

• Request count

IMPORTANT: Measure on real devices with real network conditions. Desktop Chrome with fast connection isn't representative.

NEVER:

• Optimize without measuring (premature optimization)

• Sacrifice accessibility for performance

• Break functionality while optimizing

• Use will-change everywhere (creates new layers, uses memory)

• Lazy load above-fold content

• Optimize micro-optimizations while ignoring major issues (optimize the biggest bottleneck first)

• Forget about mobile performance (often slower devices, slower connections)

Verify Improvements

Test that optimizations worked:

• Before/after metrics: Compare Lighthouse scores

• Real user monitoring: Track improvements for real users

• Different devices: Test on low-end Android, not just flagship iPhone

• Slow connections: Throttle to 3G, test experience

• No regressions: Ensure functionality still works

• User perception: Does it feel faster?

Remember: Performance is a feature. Fast experiences feel more responsive, more polished, more professional. Optimize systematically, measure ruthlessly, and prioritize user-perceived performance.