Core Web Vitals
Google's Core Web Vitals are critical metrics for user experience and SEO. Optimizing these metrics directly impacts your site's rankings and user engagement.
LCP (Largest Contentful Paint)
Time when the largest visible content element renders. Measure loading performance.
Good: <2.5s | Fair: 2.5-4s | Poor: >4s
Optimize: lazy load images, defer non-critical CSS, minimize JavaScript.
FID (First Input Delay)
Time from user interaction to browser response. Measure interactivity.
Good: <100ms | Fair: 100-300ms | Poor: >300ms
Optimize: break up long tasks, reduce JavaScript, use Web Workers.
CLS (Cumulative Layout Shift)
Unexpected layout shifts during page load. Measure visual stability.
Good: <0.1 | Fair: 0.1-0.25 | Poor: >0.25
Optimize: reserve space for ads/images, avoid inserting content above existing.
Bundle Size Optimization
Minification & Compression
- ✓ Minify JavaScript with Terser or esbuild
- ✓ Remove unused CSS with PurgeCSS/Tailwind
- ✓ Enable gzip/brotli compression on servers
- ✓ Can reduce bundle by 50-70%
Tree Shaking
- ✓ Remove dead code from bundles
- ✓ Use ES6 modules for better shaking
- ✓ Avoid CommonJS when possible
- ✓ Webpack/Rollup support tree shaking natively
Library Alternatives
- moment.js (26KB) → date-fns (13KB)
- lodash (71KB) → lodash-es + tree-shake (5-10KB)
- jQuery → native DOM APIs (0KB)
- Check bundlephobia.com for size impact
Rendering Performance
Async & Defer Scripts
<script async> - Download in parallel, execute ASAP
<script defer> - Download in parallel, execute in order
Virtual Scrolling
Only render visible list items. Dramatically improves large list performance.
Debounce & Throttle
Limit event handler execution for scroll, resize, search events.
requestAnimationFrame
Use for animations and smooth DOM updates. Syncs with browser refresh rate.
CSS containment
Use contain property to isolate component rendering impact.
Memory Management
Prevent Memory Leaks
- ✓ Remove event listeners when done
- ✓ Clean up timers (setInterval, setTimeout)
- ✓ Detach DOM nodes before removing
- ✓ Avoid circular references
- ✓ Use WeakMap for object associations
Monitoring Memory
- ✓ Chrome DevTools Memory tab
- ✓ Heap snapshots to find leaks
- ✓ Memory profiler for performance
- ✓ Performance.memory API
Code Splitting
Split code into smaller chunks. Load only what's needed for each page or route. Critical for large applications.
Dynamic Imports
import('/heavy-module.js')
.then(module => module.init())
Route-Based Splitting (React)
const Dashboard = lazy(() =>
import('./pages/Dashboard')
)
Bundle Analyzer
- ✓ webpack-bundle-analyzer
- ✓ source-map-explorer
- ✓ bundlesize
Caching Strategies
HTTP Caching
Cache-Control headers for browser and CDN caching. Huge performance impact.
Service Workers
Cache assets offline. Perfect for PWAs and offline-first apps.
In-Memory Caching
Cache API responses and computations in JavaScript. Use with careful cleanup.
CDN Caching
Serve static assets from edge locations. Massive latency reduction.
Performance Monitoring
Performance API
const paint = performance.getEntriesByType('paint')
const metrics = web-vitals.getCLS()
Tools & Services
- ✓ Google PageSpeed Insights
- ✓ Lighthouse (Chrome)
- ✓ WebPageTest
- ✓ Sentry for RUM
- ✓ New Relic for monitoring
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