---

name: performance-optimization description: Guides performance analysis and optimization for any application. Use when diagnosing slowness, optimizing code, improving load times, or when asked about performance.

Performance Optimization Skill

Performance Analysis Process

1. Measure First

Never optimize without data. Always profile before changing code.

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

# Python profiling
python -m cProfile -o profile.stats app.py
python -m pstats profile.stats

# Web performance
lighthouse https://example.com --output=json

2. Identify Bottlenecks

Common Bottleneck Categories

Category	Symptoms	Tools
CPU	High CPU usage, slow computation	Profiler, flame graphs
Memory	High RAM, GC pauses, OOM	Heap snapshots, memory profiler
I/O	Slow disk/network, waiting	strace, network inspector
Database	Slow queries, lock contention	Query analyzer, EXPLAIN

3. Apply Optimizations

Frontend Optimizations

Bundle Size

// ❌ Import entire library
import _ from 'lodash';

// ✅ Import only needed functions
import debounce from 'lodash/debounce';

// ✅ Use dynamic imports for code splitting
const HeavyComponent = lazy(() => import('./HeavyComponent'));

Rendering

// ❌ Render on every parent update
function Child({ data }) {
  return <ExpensiveComponent data={data} />;
}

// ✅ Memoize when props don't change
const Child = memo(function Child({ data }) {
  return <ExpensiveComponent data={data} />;
});

// ✅ Use useMemo for expensive computations
const processed = useMemo(() => expensiveCalc(data), [data]);

Images

<!-- ❌ Unoptimized -->
<img src="large-image.jpg" />

<!-- ✅ Optimized -->
<img 
  src="image.webp"
  srcset="image-300.webp 300w, image-600.webp 600w"
  sizes="(max-width: 600px) 300px, 600px"
  loading="lazy"
  decoding="async"
/>

Backend Optimizations

Database Queries

-- ❌ N+1 Query Problem
SELECT * FROM users;
-- Then for each user:
SELECT * FROM orders WHERE user_id = ?;

-- ✅ Single query with JOIN
SELECT u.*, o.* 
FROM users u 
LEFT JOIN orders o ON u.id = o.user_id;

-- ✅ Or use pagination
SELECT * FROM users LIMIT 100 OFFSET 0;

Caching Strategy

// Multi-layer caching
const getUser = async (id) => {
  // L1: In-memory cache (fastest)
  let user = memoryCache.get(`user:${id}`);
  if (user) return user;
  
  // L2: Redis cache (fast)
  user = await redis.get(`user:${id}`);
  if (user) {
    memoryCache.set(`user:${id}`, user, 60);
    return JSON.parse(user);
  }
  
  // L3: Database (slow)
  user = await db.users.findById(id);
  await redis.setex(`user:${id}`, 3600, JSON.stringify(user));
  memoryCache.set(`user:${id}`, user, 60);
  
  return user;
};

Async Processing

// ❌ Blocking operation
app.post('/upload', async (req, res) => {
  await processVideo(req.file);  // Takes 5 minutes
  res.send('Done');
});

// ✅ Queue for background processing
app.post('/upload', async (req, res) => {
  const jobId = await queue.add('processVideo', { file: req.file });
  res.send({ jobId, status: 'processing' });
});

Algorithm Optimizations

Time Complexity Improvements

// ❌ 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;
}

// ✅ O(n) - hash map
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 [...duplicates];
}

Performance Metrics

Web Vitals (Target Values)

Metric	Good	Needs Work	Poor
LCP	< 2.5s	2.5-4s	> 4s
FID	< 100ms	100-300ms	> 300ms
CLS	< 0.1	0.1-0.25	> 0.25
TTFB	< 800ms	800ms-1.8s	> 1.8s

API Performance (Target Values)

Metric	Target
P50 Latency	< 100ms
P95 Latency	< 500ms
P99 Latency	< 1s
Error Rate	< 0.1%