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.
facebook
langgenius
google-gemini
moltbot