# Speed Index (SI)

> Speed Index is a Lighthouse lab metric that shows how quickly the visible content of a page fills in during load. Lower is better.

**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](/glossary/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](/glossary/fcp) and LCP: faster server responses, fewer render-blocking resources and smaller above-the-fold images.