Performance Profiler Skill

Performance Profiler

Instructions

When profiling performance:

Identify the bottleneck type: Network, rendering, memory, or compute
Measure baseline before optimizing
Profile with appropriate tools
Apply optimizations
Measure improvement

Web Performance

Core Web Vitals

# Lighthouse CLI
npx lighthouse https://yoursite.com --view

# With specific metrics
npx lighthouse https://yoursite.com --only-categories=performance

Target Metrics: | Metric | Good | Needs Work | Poor | |--------|------|------------|------| | LCP (Largest Contentful Paint) | < 2.5s | 2.5-4s | > 4s | | INP (Interaction to Next Paint) | < 200ms | 200-500ms | > 500ms | | CLS (Cumulative Layout Shift) | < 0.1 | 0.1-0.25 | > 0.25 |

Bundle Analysis

# Next.js
ANALYZE=true npm run build

# Webpack
npx webpack-bundle-analyzer stats.json

# Vite
npx vite-bundle-visualizer

React Performance

React DevTools Profiler

Install React DevTools browser extension
Open DevTools → Profiler tab
Click Record, interact with app, stop recording
Analyze flame graph for slow components

Common React Optimizations

// 1. Memoize expensive components
const MemoizedList = React.memo(function List({ items }) {
  return items.map(item => <Item key={item.id} {...item} />);
});

// 2. Use useMemo for expensive calculations
const sortedItems = useMemo(() => {
  return [...items].sort((a, b) => a.name.localeCompare(b.name));
}, [items]);

// 3. Use useCallback for stable function references
const handleClick = useCallback((id: string) => {
  setSelected(id);
}, []);

// 4. Virtualize long lists
import { FixedSizeList } from 'react-window';

function VirtualList({ items }) {
  return (
    <FixedSizeList
      height={400}
      itemCount={items.length}
      itemSize={50}
      width="100%"
    >
      {({ index, style }) => (
        <div style={style}>{items[index].name}</div>
      )}
    </FixedSizeList>
  );
}

// 5. Lazy load components
const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <HeavyComponent />
    </Suspense>
  );
}

Node.js Performance

Profiling

# CPU profile
node --prof app.js
node --prof-process isolate-*.log > profile.txt

# Heap snapshot
node --inspect app.js
# Then use Chrome DevTools Memory tab

# Clinic.js (comprehensive)
npx clinic doctor -- node app.js
npx clinic flame -- node app.js
npx clinic bubbleprof -- node app.js

Memory Leak Detection

// Add to app for debugging
const used = process.memoryUsage();
console.log({
  heapUsed: `${Math.round(used.heapUsed / 1024 / 1024)} MB`,
  heapTotal: `${Math.round(used.heapTotal / 1024 / 1024)} MB`,
  external: `${Math.round(used.external / 1024 / 1024)} MB`,
});

Database Performance

-- PostgreSQL: Analyze slow queries
EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'test@example.com';

-- Find missing indexes
SELECT relname, seq_scan, idx_scan
FROM pg_stat_user_tables
WHERE seq_scan > idx_scan;

Query Optimization

// Bad: N+1 query
const users = await db.user.findMany();
for (const user of users) {
  const posts = await db.post.findMany({ where: { userId: user.id } });
}

// Good: Single query with include
const users = await db.user.findMany({
  include: { posts: true }
});

// Good: Select only needed fields
const users = await db.user.findMany({
  select: { id: true, name: true, email: true }
});

Quick Wins Checklist

[ ] Enable gzip/brotli compression
[ ] Add caching headers
[ ] Lazy load images (loading="lazy")
[ ] Preconnect to external domains
[ ] Use CDN for static assets
[ ] Minimize JavaScript bundle
[ ] Defer non-critical JS
[ ] Optimize images (WebP, proper sizing)
[ ] Add database indexes
[ ] Use connection pooling

Agent Skills: Performance Profiler

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