metrics
LCP (Largest Contentful Paint)
Time until the largest above-the-fold element renders — under 2.5s is good, over 4s fails Core Web Vitals.
Definition
Largest Contentful Paint measures the moment the largest visible image or text block in the viewport is rendered. It's the user's perceptual moment of 'the page loaded'. Good < 2.5s; needs improvement 2.5–4s; poor > 4s. Slow LCP excludes pages from AI Overview eligibility and degrades retrieval ranking.
Example
A hero image of 600KB on a slow CDN at 4.2s LCP = poor. Same image preloaded + WebP-compressed at 1.8s LCP = good.How to optimize
Preload the LCP image. Use modern formats (WebP/AVIF). Serve from edge CDN. Drop render-blocking JS. Measure with Lighthouse + CrUX.