Largest Contentful Paint (LCP) measures the render time of the largest image, text block, or video element visible within the viewport, relative to when the page first started loading. It captures perceived load speed: the moment the user feels the page is "there".
Google's LCP thresholds at the 75th percentile of page visits: under 2.5 seconds is "Good", 2.5 to 4.0 seconds is "Needs Improvement", over 4.0 is "Poor". A poor LCP triggers a Core Web Vitals failure in Search Console and is a confirmed Google Search ranking signal.
Common causes of poor LCP: slow server response (high TTFB), render-blocking JavaScript or CSS, large unoptimized images, and missing priority hints on the LCP image in modern frameworks like Next.js.