---

name: performance-optimizer description: Expert in React performance optimization, code splitting, lazy loading, memoization, bundle analysis, Core Web Vitals, Lighthouse audits, and runtime performance profiling allowed-tools:

• Read
• Write
• Edit
• Bash
• Glob
• Grep
• Task

---

Performance Optimizer

Expert skill for optimizing React application performance. Specializes in code splitting, bundle optimization, memoization, Core Web Vitals, and runtime profiling.

Core Capabilities

1. Code Splitting

• React.lazy(): Component-level splitting
• Dynamic import(): Route-based splitting
• Loadable Components: Advanced code splitting
• Bundle Analysis: webpack-bundle-analyzer
• Tree Shaking: Remove unused code

2. React Performance

• React.memo(): Prevent unnecessary renders
• useMemo(): Memoize expensive computations
• useCallback(): Memoize functions
• useTransition(): Non-blocking updates
• useDeferredValue(): Defer non-urgent updates
• React Profiler: Measure render performance

3. Bundle Optimization

• Minification: Terser, SWC
• Compression: Gzip, Brotli
• Source Maps: Optimize for production
• Dead Code Elimination: Remove unused exports
• Module Concatenation: Scope hoisting
• Long-term Caching: Content hashing

4. Image Optimization

• WebP/AVIF: Modern formats
• Responsive Images: srcset, sizes
• Lazy Loading: loading="lazy"
• Image CDN: Cloudinary, Imgix
• Placeholder: BlurHash, LQIP
• Compression: Quality optimization

5. Core Web Vitals

• LCP: Largest Contentful Paint < 2.5s
• FID: First Input Delay < 100ms
• CLS: Cumulative Layout Shift < 0.1
• TTFB: Time to First Byte < 800ms
• FCP: First Contentful Paint < 1.8s

6. Runtime Performance

• Virtual Lists: react-window, react-virtualized
• Debouncing: Limit function calls
• Throttling: Rate limiting
• Web Workers: Background processing
• Intersection Observer: Lazy load visibility
• RAF: requestAnimationFrame

Code Splitting

Copy// Route-based splitting
import { lazy, Suspense } from 'react'
import { BrowserRouter, Routes, Route } from 'react-router-dom'

const Home = lazy(() => import('./pages/Home'))
const Dashboard = lazy(() => import('./pages/Dashboard'))
const Profile = lazy(() => import('./pages/Profile'))

function App() {
  return (
    <BrowserRouter>
      <Suspense fallback={<div>Loading...</div>}>
        <Routes>
          <Route path="/" element={<Home />} />
          <Route path="/dashboard" element={<Dashboard />} />
          <Route path="/profile" element={<Profile />} />
        </Routes>
      </Suspense>
    </BrowserRouter>
  )
}

// Component-level splitting
const HeavyChart = lazy(() => import('./components/HeavyChart'))

function Analytics() {
  const [showChart, setShowChart] = useState(false)

  return (
    <div>
      <button onClick={() => setShowChart(true)}>Show Chart</button>
      {showChart && (
        <Suspense fallback={<div>Loading chart...</div>}>
          <HeavyChart />
        </Suspense>
      )}
    </div>
  )
}

Memoization

Copy// React.memo for components
const ExpensiveComponent = React.memo(({ data }: { data: Data }) => {
  return <div>{/* Expensive rendering */}</div>
})

// useMemo for values
function SearchResults({ query }: { query: string }) {
  const filteredResults = useMemo(() => {
    return expensiveFilter(items, query)
  }, [items, query])

  return <div>{/* Render results */}</div>
}

// useCallback for functions
function Parent() {
  const handleClick = useCallback(() => {
    console.log('Clicked')
  }, [])

  return <Child onClick={handleClick} />
}

Virtual Lists

Copyimport { FixedSizeList } from 'react-window'

function VirtualList({ items }: { items: Item[] }) {
  const Row = ({ index, style }: { index: number; style: React.CSSProperties }) => (
    <div style={style}>{items[index].name}</div>
  )

  return (
    <FixedSizeList
      height={600}
      itemCount={items.length}
      itemSize={50}
      width="100%"
    >
      {Row}
    </FixedSizeList>
  )
}

Bundle Analysis

Copy// webpack.config.js
const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPlugin

module.exports = {
  plugins: [
    new BundleAnalyzerPlugin({
      analyzerMode: 'static',
      openAnalyzer: false,
    }),
  ],
}

// package.json
{
  "scripts": {
    "analyze": "source-map-explorer 'build/static/js/*.js'"
  }
}

Image Optimization

Copy// Responsive + Lazy Loading
function OptimizedImage({ src, alt }: { src: string; alt: string }) {
  return (
    <img
      src={src}
      alt={alt}
      srcSet={`
        ${src}?w=400 400w,
        ${src}?w=800 800w,
        ${src}?w=1200 1200w
      `}
      sizes="(max-width: 768px) 100vw, 50vw"
      loading="lazy"
      decoding="async"
      style={{ width: '100%', height: 'auto' }}
    />
  )
}

Core Web Vitals

Copy// Measure Web Vitals
import { getCLS, getFID, getFCP, getLCP, getTTFB } from 'web-vitals'

function sendToAnalytics(metric: any) {
  console.log(metric)
  // Send to analytics
}

getCLS(sendToAnalytics)
getFID(sendToAnalytics)
getFCP(sendToAnalytics)
getLCP(sendToAnalytics)
getTTFB(sendToAnalytics)

Best Practices

• Lazy load below-the-fold content
• Use production builds
• Enable compression (Gzip/Brotli)
• Optimize images (WebP, lazy loading)
• Code split by route
• Memoize expensive components
• Use virtual lists for large datasets
• Prefetch critical resources
• Monitor Core Web Vitals
• Regular bundle audits

When to Use This Skill

Use when you need to:

• Improve app load time
• Reduce bundle size
• Optimize runtime performance
• Fix Core Web Vitals issues
• Implement code splitting
• Add lazy loading
• Optimize images
• Profile React performance

Output Format

Provide:

1. Performance Audit: Current metrics
2. Optimization Plan: What to optimize
3. Implementation: Code changes
4. Bundle Analysis: Size improvements
5. Metrics: Before/after comparison
6. Monitoring: Track performance over time