performance-profiling

Performance Profiling Skill

Finds real performance bottlenecks using data. Never optimize without measuring first.

When to Activate

Activate when:

• Users report slow responses
• A page or endpoint takes longer than expected
• Before a production deployment of performance-sensitive changes
• After adding features to a hot code path

Core Principles

• Measure before optimizing — a guess about the bottleneck is almost always wrong
• Profile the real bottleneck — top-line metrics first, then drill down
• Verify improvement with numbers — "feels faster" is not a performance result

Workflow

1. Establish baseline — measure current performance with actual numbers
2. Profile — find where time is spent
3. Identify bottleneck — the one slowest thing
4. Fix — targeted change to address bottleneck
5. Measure improvement — confirm the number improved

Profiling Tools

# Node.js CPU profiling
node --prof app.js
node --prof-process isolate-*.log | head -100

# Clinic.js (visual profiler)
npm install -g clinic
clinic doctor -- node app.js
clinic flame -- node app.js   # flame graph

# Lighthouse (web performance)
npx lighthouse http://localhost:3000 --output=json --output-path=./report.json

# Bundle analyzer
npx webpack-bundle-analyzer dist/stats.json

Core Web Vitals Targets

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

N+1 Detection

// ❌ N+1 — 1 query for posts, N queries for authors
const posts = await Post.findAll();
for (const post of posts) {
  post.author = await User.findById(post.authorId); // N queries!
}

// ✅ Single query with JOIN
const posts = await Post.findAll({
  include: [{ model: User, as: 'author' }]
});

Detection: add query logging and look for repeated queries with different IDs.

Bundle Analysis