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:

// 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:

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

Database:

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

// 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

-- 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 *

// 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

// 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

// 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

// 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

// 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

// 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

// Bad: Imports entire library
import _ from 'lodash'; // 70KB

// Good: Import only what you need
import debounce from 'lodash/debounce'; // 2KB

Problem: No Code Splitting

// Bad: Everything in one bundle
import HeavyComponent from './HeavyComponent';

// Good: Lazy load
const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

Problem: Unoptimized Images

<!-- 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

// 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

// 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

// 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

// 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:

// 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:

• Add database indexes on frequently queried columns
• Enable gzip compression on server
• Add caching for expensive operations
• Lazy load images and heavy components
• Use CDN for static assets
• Minify and compress JavaScript/CSS
• Remove unused dependencies
• Use pagination instead of loading all data
• Optimize images (WebP, proper sizing)
• 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