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.
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.