performance-profiling

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 |

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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) |

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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 |

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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 |

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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 |

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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 |

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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 |

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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 |

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

When to Use

This skill is applicable to execute the workflow or actions described in the overview.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.