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+---
+name: performance-optimizer
+description: "Identifies and fixes performance bottlenecks in code, databases, and APIs. Measures before and after to prove improvements."
+category: development
+risk: safe
+source: community
+date_added: "2026-03-05"
+---
+
+# Performance Optimizer
+
+Find and fix performance bottlenecks. Measure, optimize, verify. Make it fast.
+
+## When to Use This Skill
+
+- App is slow or laggy
+- User complains about performance
+- Page load times are high
+- API responses are slow
+- Database queries take too long
+- User mentions "slow", "lag", "performance", or "optimize"
+
+## The Optimization Process
+
+### 1. Measure First
+
+Never optimize without measuring:
+
+```javascript
+// Measure execution time
+console.time('operation');
+await slowOperation();
+console.timeEnd('operation'); // operation: 2341ms
+```
+
+**What to measure:**
+- Page load time
+- API response time
+- Database query time
+- Function execution time
+- Memory usage
+- Network requests
+
+### 2. Find the Bottleneck
+
+Use profiling tools to find the slow parts:
+
+**Browser:**
+```
+DevTools → Performance tab → Record → Stop
+Look for long tasks (red bars)
+```
+
+**Node.js:**
+```bash
+node --prof app.js
+node --prof-process isolate-*.log > profile.txt
+```
+
+**Database:**
+```sql
+EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'test@example.com';
+```
+
+### 3. Optimize
+
+Fix the slowest thing first (biggest impact).
+
+## Common Optimizations
+
+### Database Queries
+
+**Problem: N+1 Queries**
+```javascript
+// Bad: N+1 queries
+const users = await db.users.find();
+for (const user of users) {
+  user.posts = await db.posts.find({ userId: user.id }); // N queries
+}
+
+// Good: Single query with JOIN
+const users = await db.users.find()
+  .populate('posts'); // 1 query
+```
+
+**Problem: Missing Index**
+```sql
+-- Check slow query
+EXPLAIN SELECT * FROM users WHERE email = 'test@example.com';
+-- Shows: Seq Scan (bad)
+
+-- Add index
+CREATE INDEX idx_users_email ON users(email);
+
+-- Check again
+EXPLAIN SELECT * FROM users WHERE email = 'test@example.com';
+-- Shows: Index Scan (good)
+```
+
+**Problem: SELECT ***
+```javascript
+// Bad: Fetches all columns
+const users = await db.query('SELECT * FROM users');
+
+// Good: Only needed columns
+const users = await db.query('SELECT id, name, email FROM users');
+```
+
+**Problem: No Pagination**
+```javascript
+// Bad: Returns all records
+const users = await db.users.find();
+
+// Good: Paginated
+const users = await db.users.find()
+  .limit(20)
+  .skip((page - 1) * 20);
+```
+
+### API Performance
+
+**Problem: No Caching**
+```javascript
+// Bad: Hits database every time
+app.get('/api/stats', async (req, res) => {
+  const stats = await db.stats.calculate(); // Slow
+  res.json(stats);
+});
+
+// Good: Cache for 5 minutes
+const cache = new Map();
+app.get('/api/stats', async (req, res) => {
+  const cached = cache.get('stats');
+  if (cached && Date.now() - cached.time < 300000) {
+    return res.json(cached.data);
+  }
+  
+  const stats = await db.stats.calculate();
+  cache.set('stats', { data: stats, time: Date.now() });
+  res.json(stats);
+});
+```
+
+**Problem: Sequential Operations**
+```javascript
+// Bad: Sequential (slow)
+const user = await getUser(id);
+const posts = await getPosts(id);
+const comments = await getComments(id);
+// Total: 300ms + 200ms + 150ms = 650ms
+
+// Good: Parallel (fast)
+const [user, posts, comments] = await Promise.all([
+  getUser(id),
+  getPosts(id),
+  getComments(id)
+]);
+// Total: max(300ms, 200ms, 150ms) = 300ms
+```
+
+**Problem: Large Payloads**
+```javascript
+// Bad: Returns everything
+res.json(users); // 5MB response
+
+// Good: Only needed fields
+res.json(users.map(u => ({
+  id: u.id,
+  name: u.name,
+  email: u.email
+}))); // 500KB response
+```
+
+### Frontend Performance
+
+**Problem: Unnecessary Re-renders**
+```javascript
+// Bad: Re-renders on every parent update
+function UserList({ users }) {
+  return users.map(user => <UserCard user={user} />);
+}
+
+// Good: Memoized
+const UserCard = React.memo(({ user }) => {
+  return <div>{user.name}</div>;
+});
+```
+
+**Problem: Large Bundle**
+```javascript
+// Bad: Imports entire library
+import _ from 'lodash'; // 70KB
+
+// Good: Import only what you need
+import debounce from 'lodash/debounce'; // 2KB
+```
+
+**Problem: No Code Splitting**
+```javascript
+// Bad: Everything in one bundle
+import HeavyComponent from './HeavyComponent';
+
+// Good: Lazy load
+const HeavyComponent = React.lazy(() => import('./HeavyComponent'));
+```
+
+**Problem: Unoptimized Images**
+```html
+<!-- Bad: Large image -->
+<img src="photo.jpg" /> <!-- 5MB -->
+
+<!-- Good: Optimized and responsive -->
+<img 
+  src="photo-small.webp" 
+  srcset="photo-small.webp 400w, photo-large.webp 800w"
+  loading="lazy"
+  width="400"
+  height="300"
+/> <!-- 50KB -->
+```
+
+### Algorithm Optimization
+
+**Problem: Inefficient Algorithm**
+```javascript
+// Bad: O(n²) - nested loops
+function findDuplicates(arr) {
+  const duplicates = [];
+  for (let i = 0; i < arr.length; i++) {
+    for (let j = i + 1; j < arr.length; j++) {
+      if (arr[i] === arr[j]) duplicates.push(arr[i]);
+    }
+  }
+  return duplicates;
+}
+
+// Good: O(n) - single pass with Set
+function findDuplicates(arr) {
+  const seen = new Set();
+  const duplicates = new Set();
+  for (const item of arr) {
+    if (seen.has(item)) duplicates.add(item);
+    seen.add(item);
+  }
+  return Array.from(duplicates);
+}
+```
+
+**Problem: Repeated Calculations**
+```javascript
+// Bad: Calculates every time
+function getTotal(items) {
+  return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
+}
+// Called 100 times in render
+
+// Good: Memoized
+const getTotal = useMemo(() => {
+  return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
+}, [items]);
+```
+
+### Memory Optimization
+
+**Problem: Memory Leak**
+```javascript
+// Bad: Event listener not cleaned up
+useEffect(() => {
+  window.addEventListener('scroll', handleScroll);
+  // Memory leak!
+}, []);
+
+// Good: Cleanup
+useEffect(() => {
+  window.addEventListener('scroll', handleScroll);
+  return () => window.removeEventListener('scroll', handleScroll);
+}, []);
+```
+
+**Problem: Large Data in Memory**
+```javascript
+// Bad: Loads entire file into memory
+const data = fs.readFileSync('huge-file.txt'); // 1GB
+
+// Good: Stream it
+const stream = fs.createReadStream('huge-file.txt');
+stream.on('data', chunk => process(chunk));
+```
+
+## Measuring Impact
+
+Always measure before and after:
+
+```javascript
+// Before optimization
+console.time('query');
+const users = await db.users.find();
+console.timeEnd('query');
+// query: 2341ms
+
+// After optimization (added index)
+console.time('query');
+const users = await db.users.find();
+console.timeEnd('query');
+// query: 23ms
+
+// Improvement: 100x faster!
+```
+
+## Performance Budgets
+
+Set targets:
+
+```
+Page Load: < 2 seconds
+API Response: < 200ms
+Database Query: < 50ms
+Bundle Size: < 200KB
+Time to Interactive: < 3 seconds
+```
+
+## Tools
+
+**Browser:**
+- Chrome DevTools Performance tab
+- Lighthouse (audit)
+- Network tab (waterfall)
+
+**Node.js:**
+- `node --prof` (profiling)
+- `clinic` (diagnostics)
+- `autocannon` (load testing)
+
+**Database:**
+- `EXPLAIN ANALYZE` (query plans)
+- Slow query log
+- Database profiler
+
+**Monitoring:**
+- New Relic
+- Datadog
+- Sentry Performance
+
+## Quick Wins
+
+Easy optimizations with big impact:
+
+1. **Add database indexes** on frequently queried columns
+2. **Enable gzip compression** on server
+3. **Add caching** for expensive operations
+4. **Lazy load** images and heavy components
+5. **Use CDN** for static assets
+6. **Minify and compress** JavaScript/CSS
+7. **Remove unused dependencies**
+8. **Use pagination** instead of loading all data
+9. **Optimize images** (WebP, proper sizing)
+10. **Enable HTTP/2** on server
+
+## Optimization Checklist
+
+- [ ] Measured current performance
+- [ ] Identified bottleneck
+- [ ] Applied optimization
+- [ ] Measured improvement
+- [ ] Verified functionality still works
+- [ ] No new bugs introduced
+- [ ] Documented the change
+
+## When NOT to Optimize
+
+- Premature optimization (optimize when it's actually slow)
+- Micro-optimizations (save 1ms when page takes 5 seconds)
+- Readable code is more important than tiny speed gains
+- If it's already fast enough
+
+## Key Principles
+
+- Measure before optimizing
+- Fix the biggest bottleneck first
+- Measure after to prove improvement
+- Don't sacrifice readability for tiny gains
+- Profile in production-like environment
+- Consider the 80/20 rule (20% of code causes 80% of slowness)
+
+## Related Skills
+
+- `@database-design` - Query optimization
+- `@codebase-audit-pre-push` - Code review
+- `@bug-hunter` - Debugging