performance

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• SKILL.md

SKILL.md (first lines)

---
name: performance
description: Optimize web performance for faster loading and better user experience. Use when asked to "speed up my site", "optimize performance", "reduce load time", "fix slow loading", "improve page speed", or "performance audit".
license: MIT
metadata:
  author: web-quality-skills
  version: "1.0"
---

# Performance optimization

Deep performance optimization based on Lighthouse performance audits. Focuses on loading speed, runtime efficiency, and resource optimization.

## How it works

1. Identify performance bottlenecks in code and assets
2. Prioritize by impact on Core Web Vitals
3. Provide specific optimizations with code examples
4. Measure improvement with before/after metrics

## Performance budget

| Resource | Budget | Rationale |
|----------|--------|-----------|
| Total page weight | < 1.5 MB | 3G loads in ~4s |
| JavaScript (compressed) | < 300 KB | Parsing + execution time |
| CSS (compressed) | < 100 KB | Render blocking |
| Images (above-fold) | < 500 KB | LCP impact |
| Fonts | < 100 KB | FOIT/FOUT prevention |
| Third-party | < 200 KB | Uncontrolled latency |

## Critical rendering path

### Server response
* **TTFB < 800ms.** Time to First Byte should be fast. Use CDN, caching, and efficient backends.
* **Enable compression.** Gzip or Brotli for text assets. Brotli preferred (15-20% smaller).
* **HTTP/2 or HTTP/3.** Multiplexing reduces connection overhead.
* **Edge caching.** Cache HTML at CDN edge when possible.

### Resource loading

**Preconnect to required origins:**
```html
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com" crossorigin>
```

**Preload critical resources:**
```html
<!-- LCP image -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">

<!-- Critical font -->
<link rel="preload" href="/font.woff2" as="font" type="font/woff2" crossorigin>
```

**Defer non-critical CSS:**
```html
<!-- Critical CSS inlined -->
<style>/* Above-fold styles */</style>

<!-- Non-critical CSS -->
<link rel="preload" href="/styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="/styles.css"></noscript>
```

### JavaScript optimization

**Defer non-essential scripts:**
```html
<!-- Parser-blocking (avoid) -->
<script src="/critical.js"></script>

<!-- Deferred (preferred) -->
<script defer src="/app.js"></script>

<!-- Async (for independent scripts) -->
<script async src="/analytics.js"></script>

<!-- Module (deferred by default) -->
<script type="module" src="/app.mjs"></script>
```

**Code splitting patterns:**
```javascript
// Route-based splitting
const Dashboard = lazy(() => import('./Dashboard'));

// Component-based splitting
const HeavyChart = lazy(() => import('./HeavyChart'));

// Feature-based splitting
if (user.isPremium) {
  const PremiumFeatures = await import('./PremiumFeatures');
}
```

**Tree shaking best practices:**
```javascript
// ❌ Imports entire library
import _ from 'lodash';
_.debounce(fn, 300);

// ✅ Imports only what's needed
import debounce from 'lodash/debounce';
debounce(fn, 300);
```

## Image optimization

### Format selection
| Format | Use case | Browser support |
|--------|----------|-----------------|
| AVIF | Photos, best compression | 92%+ |
| WebP | Photos, good fallback | 97%+ |
| PNG | Graphics with transparency | Universal |

From Testing-itay/DevTools-Security-Lab, MIT.

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