Performance Profiling
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.