First Contentful Paint (FCP) measures the time from navigation start to when the browser first renders any DOM content -- text, an image, or a non-blank canvas. It captures the moment the user knows "something is happening" on the page.
FCP is part of Lighthouse's performance score (10% weight) but is NOT a Core Web Vital. Google's thresholds: under 1.8 seconds is "Good", 1.8 to 3.0 seconds is "Needs Improvement", over 3.0 is "Poor".
Optimisations that improve FCP: eliminate render-blocking resources (defer non-critical JS, inline critical CSS), preconnect to required origins, use a CDN, and ensure the HTML response itself isn't slow (TTFB under 600ms).