Speed Index

Speed Index is the average time it takes for the visible parts of the page to be filled in; the faster and more evenly content appears, the lower the number.

Speed Index differs fundamentally from the other metrics: instead of a single moment (like FCP or LCP), it measures the whole progression of the picture filling in over time, so a page that appears in laggy chunks scores a poor Speed Index even with an acceptable LCP. For this reason it is a lab metric: computing it needs video frames of the load, produced only in a controlled environment such as Lighthouse, not from a real user. Its number should be read as a qualitative lab band, not a standardized field threshold; in Lighthouse's mobile scoring, roughly under 3.4 s counts as good. Its worth is that it puts a number on “how full and complete the page feels,” something a single-moment metric misses.