Web Performance Optimization: Core Web Vitals in 2025

Performance directly impacts user experience and SEO. Here's how to ace Core Web Vitals in 2025. ## Understanding the Three Metrics **LCP (Largest Contentful Paint)** measures loading performance. Target: under 2.5s. **INP (Interaction to Next Paint)** replaced FID — it measures responsiveness to all interactions. Target: under 200ms. **CLS (Cumulative Layout Shift)** measures visual stability. Target: under 0.1. ## Optimizing LCP The LCP element is usually a hero image or heading. To optimize it: - Use `<link rel="preload">` for the hero image - Add `fetchpriority="high"` to the LCP image - Serve images in WebP or AVIF format - Use a CDN ```html <link rel="preload" as="image" href="/hero.webp" fetchpriority="high" /> ``` ## Reducing INP Long tasks block the main thread. Break them up: - Use `scheduler.yield()` to yield back to the browser - Virtualize long lists with `react-window` or `@tanstack/virtual` - Debounce expensive event handlers ## Preventing CLS - Always set explicit `width` and `height` on images - Reserve space for ads and embeds - Avoid inserting content above existing content - Use `font-display: optional` or `swap` carefully ## Tools - Lighthouse in Chrome DevTools - PageSpeed Insights - Chrome User Experience Report (CrUX) - WebPageTest ## Conclusion Performance optimization is an ongoing process. Measure first, then optimize the biggest wins. Small improvements compound into significantly better user experiences.