Speed Index (SI) measures how quickly content is visually displayed during page load. Lighthouse records the load frame by frame and calculates how fast the viewport fills in. Two pages with the same LCP can have very different Speed Index values if one renders progressively and the other stays blank until the end.
Speed Index accounts for 10% of the Lighthouse performance score. On mobile, roughly 3.4 seconds or less is treated as good. It improves with the same work that improves FCP and LCP: faster server responses, fewer render-blocking resources and smaller above-the-fold images.