web-performance-optimization

What this skill does

# Web Performance Optimization

Optimize web performance through Core Web Vitals, modern browser APIs (View Transitions, Speculation Rules), and framework-specific techniques.

## When to Use This Skill

- Improving Lighthouse scores (target: 90+)
- Reducing page load times (target: <2.5s LCP)
- Optimizing Core Web Vitals for SEO rankings
- Implementing modern performance patterns (2025)
- Debugging performance bottlenecks
- Setting up continuous performance monitoring

**Business Impact:** 1 second delay = 7% conversion loss. 0.1s improvement = 8% increase in conversions.

## Start Here: Quick Wins

**High-ROI optimizations by time investment:**

**1 Hour Quick Wins (⭐⭐⭐⭐⭐ ROI):**
- Add `loading="lazy"` to below-fold images (40-60% weight reduction)
- Enable compression (gzip/brotli) (70-80% transfer size reduction)
- Add `rel="preconnect"` for critical origins (100-500ms savings)

**1 Day Investments (⭐⭐⭐⭐ ROI):**
- Implement code splitting (30-50% bundle reduction)
- Optimize LCP image with `fetchpriority="high"` (200-400ms improvement)
- Add basic service worker (instant repeat visits)

**1 Week Comprehensive (⭐⭐⭐⭐⭐ ROI):**
- Full caching strategy (HTTP headers + service workers)
- Bundle optimization (tree shaking, 40-60% reduction)
- Performance monitoring (Lighthouse CI + RUM)

**→ See [quick-wins.md](./references/quick-wins.md) for complete implementation details**

## Core Web Vitals at a Glance

| Metric | Target | Weight | Key Optimization |
|--------|--------|--------|------------------|
| **LCP** (Largest Contentful Paint) | <2.5s | 25% | Optimize images, preload critical resources |
| **INP** (Interaction to Next Paint) | <200ms | 30% | Reduce JavaScript, break up long tasks |
| **CLS** (Cumulative Layout Shift) | <0.1 | 25% | Reserve space, optimize fonts |
| **TBT** (Total Blocking Time) | <200ms | 30% | Code splitting, defer non-critical JS |
| **FCP** (First Contentful Paint) | <1.8s | 10% | Eliminate render-blocking resources |

**→ See [core-web-vitals.md](./references/core-web-vitals.md) for deep dive on each metric**

## Modern Patterns (2025)

**View Transitions API** - Smooth page transitions without JavaScript
```css
@view-transition { navigation: auto; }
```

**Speculation Rules API** - Prerender pages for instant navigation  
**React Server Components** - Zero-bundle server components  
**Priority Hints** - `fetchpriority="high"` for LCP optimization  
**Content Visibility** - CSS-based rendering optimization

**→ See [modern-patterns-2025.md](./references/modern-patterns-2025.md) for cutting-edge techniques**

## Navigation

### Quick Start (Load First)
- **[⚡ Quick Wins](./references/quick-wins.md)** - Time-boxed optimizations (1hr/1day/1week) with ROI ratings. Load FIRST for immediate impact.

### Deep Dives
- **[📊 Core Web Vitals](./references/core-web-vitals.md)** - LCP, INP, CLS optimization strategies with debugging workflows. Load when targeting specific metrics.
- **[🔧 Optimization Techniques](./references/optimization-techniques.md)** - Image, JavaScript, CSS, resource loading, caching patterns. Load when implementing specific optimizations.

### Modern Features
- **[✨ Modern Patterns 2025](./references/modern-patterns-2025.md)** - View Transitions, Speculation Rules, RSC, Islands Architecture. Load when adopting cutting-edge patterns.

### Framework-Specific
- **[⚛️ Framework Patterns](./references/framework-specific.md)** - Next.js, React, Vue, Vite, Astro, SvelteKit optimizations. Load for your framework.

### Measurement & Monitoring
- **[📈 Monitoring](./references/monitoring.md)** - Lighthouse CI, RUM setup, performance budgets, debugging tools. Load when setting up continuous monitoring.

## Key Reminders

- **Measure first, optimize second** - Always baseline with Lighthouse/WebPageTest before changes
- **Focus on user-centric metrics** - Core Web Vitals > vanity metrics
- **Test on real devices** - 53% of mobile users abandon after 3 seconds
- **Monitor continuously** - Performance degrades over time without vigilance
- **Prioritize by ROI** - Start with Quick Wins (high impact, low effort)

## Red Flags

Stop and reconsider if:
- Optimizing without baseline measurement
- Focusing only on Lighthouse score (ignoring field data)
- Ignoring mobile performance (53% abandon rate after 3s)
- Loading all resources eagerly (no lazy loading)
- No caching strategy implemented
- Third-party scripts loaded synchronously
- Missing performance monitoring/budgets

## Integration with Other Skills

- **nextjs-core** - Next.js Image component, font optimization, static generation
- **react** - Component optimization, memoization, code splitting
- **vite** - Build optimization, chunk splitting
- **typescript-core** - Type-safe performance patterns

## Real-World Impact

**Conversion Impact:**
- Pinterest: 40% faster perceived wait = 15% more sign-ups + 15% more SEO traffic
- Zalando: 0.1s improvement = 0.7% revenue increase
- AutoAnything: 50% faster load = 12-13% sales increase

**SEO Impact:**
- Core Web Vitals are ranking factors (June 2021+)
- 75% of users passing Core Web Vitals = ranking boost
- Mobile-first indexing prioritizes mobile performance

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**Remember:** Performance is a feature, not an afterthought. Every millisecond counts. Start with Quick Wins for immediate impact.