Why Core Web Vitals Matter
Google uses user-experience metrics called Core Web Vitals as direct ranking signals. A fast website doesn’t just please your users; it directly increases your search engine visibility. For Single Page Applications (SPAs) built with React, Vue, or Angular, maintaining high scores requires deliberate optimization.
1. Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visual element on the page (usually a hero image or heading) to render. To optimize LCP:
- Preload critical images using
<link rel="preload">. - Decompress and compress images to modern formats like WebP or AVIF.
- Minify and defer non-critical JavaScript to prevent render blocking.
2. Cumulative Layout Shift (CLS)
CLS measures visual stability. If elements shift around on the screen during load, users get frustrated and click the wrong things.
/* Bad: Image without explicit height and width */
img { width: 100%; }
/* Good: Reserving space for the image to prevent layout shifts */
.img-container {
aspect-ratio: 16 / 9;
background: #111;
}