Speed Index

Speed Index measures how fast the content of a page is visibly populated. Lighthouse takes screenshots at intervals during page load, calculates the visual completeness at each frame, and integrates the result. Lower is better — a fast SI means the page filled in quickly even if the absolute "complete" moment came later.

A good Speed Index is under 3.4 seconds; under 1.5 seconds is excellent. Unlike LCP, SI rewards progressive rendering — a page that streams content into view section by section will score better than one that waits for everything before drawing.

SI is a Lighthouse-only metric and isn’t in the Core Web Vitals set, but designers often use it as a tiebreaker between two LCP-equivalent options. The streaming option always wins.