Performance is crucial for modern web applications. Users expect fast, responsive experiences, and search engines reward sites that deliver them.

Core Web Vitals

Google's Core Web Vitals are the foundation of web performance:

Largest Contentful Paint (LCP)

  • Target: Under 2.5 seconds
  • What it measures: Loading performance
  • How to improve: Optimize images, use CDN, minimize render-blocking resources

First Input Delay (FID)

  • Target: Under 100 milliseconds
  • What it measures: Interactivity
  • How to improve: Reduce JavaScript execution time, code splitting

Cumulative Layout Shift (CLS)

  • Target: Under 0.1
  • What it measures: Visual stability
  • How to improve: Use size attributes, avoid inserting content above fold

Modern Optimization Techniques

1. Image Optimization

<!-- Use modern formats -->
<picture>
	<source srcset="image.avif" type="image/avif" />
	<source srcset="image.webp" type="image/webp" />
	<img src="image.jpg" alt="Description" loading="lazy" />
</picture>

2. Code Splitting

// Dynamic imports for code splitting
const LazyComponent = lazy(() => import('./LazyComponent.svelte'));


// Route-based splitting
const routes = {
	'/': () => import('./Home.svelte'),
	'/about': () => import('./About.svelte')
};

3. Resource Hints

<!-- Preload critical resources -->
<link rel="preload" href="critical.css" as="style" />
<link rel="preload" href="hero-image.jpg" as="image" />


<!-- Prefetch likely navigation -->
<link rel="prefetch" href="/about" />

Performance Budget

Set clear performance budgets:

  • Bundle size: < 200KB initial load
  • Time to Interactive: < 3 seconds
  • Lighthouse score: > 90

Monitoring Tools

Development

  • Lighthouse
  • WebPageTest
  • Chrome DevTools

Production

  • Core Web Vitals report
  • Real User Monitoring (RUM)
  • Performance APIs

Best Practices

  1. Optimize Critical Rendering Path

    • Minimize render-blocking CSS
    • Defer non-critical JavaScript
    • Inline critical CSS

  2. Efficient Loading Strategies

    • Lazy load images and components
    • Use intersection observer
    • Implement progressive loading

  3. Caching Strategy

    • Browser cache headers
    • Service worker caching
    • CDN optimization

Conclusion

Performance optimization is an ongoing process. Focus on measuring real-world impact, not just synthetic scores. Remember that performance is a feature, not an afterthought.

Start with Core Web Vitals, implement monitoring, and continuously optimize based on real user data.