Speed Index (SI) measures how quickly the visible content of a page is rendered. It's calculated by capturing a video of the page load, computing the visual completeness of every frame, and integrating the unfilled portion over time. A page that renders top-to-bottom evenly scores better than one that holds a blank screen and then snaps to fully complete.
Google's Lighthouse thresholds: under 3.4 seconds is "Good", 3.4 to 5.8 is "Needs Improvement", over 5.8 is "Poor". Speed Index carries 10% weight in the Lighthouse performance score.
Speed Index improves alongside LCP and FCP -- the same optimisations apply (eliminate render-blocking resources, preload critical assets, defer non-critical scripts). It's not a Core Web Vital, but a high SI usually indicates LCP and FCP problems too.