Performance isn't optional—it's a feature. Slow sites lose users, hurt SEO, and frustrate developers. Understanding performance fundamentals helps you build fast sites from the start rather than optimizing later.
Core Web Vitals#
LCP (Largest Contentful Paint)#
Time until the largest content element is visible.
Target: < 2.5 seconds
Common culprits:
- Large unoptimized images
- Slow server response
- Render-blocking resources
- Client-side rendering
Fixes:
FID/INP (Interaction to Next Paint)#
Time from user interaction to browser response.
Target: < 200ms
Common culprits:
- Long JavaScript tasks
- Heavy main thread work
- Synchronous operations
Fixes:
CLS (Cumulative Layout Shift)#
Visual stability—how much the page shifts unexpectedly.
Target: < 0.1
Common culprits:
- Images without dimensions
- Dynamically injected content
- Web fonts causing FOUT/FOIT
Fixes:
Resource Loading#
Critical Rendering Path#
Resource Hints#
JavaScript Optimization#
Bundle Optimization#
Tree Shaking#
Avoiding Main Thread Blocking#
Image Optimization#
Modern Formats#
Responsive Images#
Image CDN Usage#
Caching Strategy#
HTTP Caching#
Service Worker Caching#
Measuring Performance#
Lab Tools#
Field Data#
Performance Budget#
Common Pitfalls#
Third-Party Scripts#
Font Loading#
Conclusion#
Web performance is a continuous practice, not a one-time fix. Understand the metrics that matter, measure consistently, and optimize deliberately. The fundamentals—efficient loading, minimal JavaScript, optimized images, smart caching—provide the foundation for fast experiences.
Start with Core Web Vitals, establish performance budgets, and make performance part of your development workflow. Fast sites win.