# Performance Profiling

> ALWAYS use this when the user mentions Performance Profiling, asks to build, debug, review, document, automate, test, configure, migrate, or make decisions in this domain, or the task clearly depends on Performance Profiling; scope: Performance profiling principles. Apply the bundled workflow, references, scripts, Senior Master standard, and Codex strict review gate before final output.

- Skill: `anhvu1107/performance-profiling` (Agent Skill, multi-file: 7 files)
- Install (CLI): `npx skillmds@latest add anhvu1107/performance-profiling`
- Raw SKILL.md: https://api.skillmd.com/api/skills/anhvu1107/performance-profiling/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: anhvu1107 (https://skillmd.com/u/anhvu1107)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/anhvu1107/performance-profiling

---


# Performance Profiling

## Selective Reading Rule

Start with:

- `references/senior-master-standard.md`
- `references/usage-routing.md`
- `references/quality-checklist.md`

Then load only the inherited docs, scripts, assets, or examples that match the user's actual task.

> Measure, analyze, optimize - in that order.

## 🔧 Runtime Scripts

**Execute these for automated profiling:**

| Script | Purpose | Usage |
|--------|---------|-------|
| `scripts/lighthouse_audit.py` | Lighthouse performance audit | `python scripts/lighthouse_audit.py https://example.com` |

---

## 1. Core Web Vitals

### Targets

| Metric | Good | Poor | Measures |
|--------|------|------|----------|
| **LCP** | < 2.5s | > 4.0s | Loading |
| **INP** | < 200ms | > 500ms | Interactivity |
| **CLS** | < 0.1 | > 0.25 | Stability |

### When to Measure

| Stage | Tool |
|-------|------|
| Development | Local Lighthouse |
| CI/CD | Lighthouse CI |
| Production | RUM (Real User Monitoring) |

---

## 2. Profiling Workflow

### The 4-Step Process

```
1. BASELINE → Measure current state
2. IDENTIFY → Find the bottleneck
3. FIX → Make targeted change
4. VALIDATE → Confirm improvement
```

### Profiling Tool Selection

| Problem | Tool |
|---------|------|
| Page load | Lighthouse |
| Bundle size | Bundle analyzer |
| Runtime | DevTools Performance |
| Memory | DevTools Memory |
| Network | DevTools Network |

---

## 3. Bundle Analysis

### What to Look For

| Issue | Indicator |
|-------|-----------|
| Large dependencies | Top of bundle |
| Duplicate code | Multiple chunks |
| Unused code | Low coverage |
| Missing splits | Single large chunk |

### Optimization Actions

| Finding | Action |
|---------|--------|
| Big library | Import specific modules |
| Duplicate deps | Dedupe, update versions |
| Route in main | Code split |
| Unused exports | Tree shake |

---

## 4. Runtime Profiling

### Performance Tab Analysis

| Pattern | Meaning |
|---------|---------|
| Long tasks (>50ms) | UI blocking |
| Many small tasks | Possible batching opportunity |
| Layout/paint | Rendering bottleneck |
| Script | JavaScript execution |

### Memory Tab Analysis

| Pattern | Meaning |
|---------|---------|
| Growing heap | Possible leak |
| Large retained | Check references |
| Detached DOM | Not cleaned up |

---

## 5. Common Bottlenecks

### By Symptom

| Symptom | Likely Cause |
|---------|--------------|
| Slow initial load | Large JS, render blocking |
| Slow interactions | Heavy event handlers |
| Jank during scroll | Layout thrashing |
| Growing memory | Leaks, retained refs |

---

## 6. Quick Win Priorities

| Priority | Action | Impact |
|----------|--------|--------|
| 1 | Enable compression | High |
| 2 | Lazy load images | High |
| 3 | Code split routes | High |
| 4 | Cache static assets | Medium |
| 5 | Optimize images | Medium |

---

## 7. Anti-Patterns

| ❌ Don't | ✅ Do |
|----------|-------|
| Guess at problems | Profile first |
| Micro-optimize | Fix biggest issue |
| Optimize early | Optimize when needed |
| Ignore real users | Use RUM data |

---

> **Remember:** The fastest code is code that doesn't run. Remove before optimizing.