Speed Index
Speed Index measures how quickly the visible part of the page fills in. Two pages that finish at the same moment can score very differently depending on how they get there.
Two pages, same finish line, different Speed Index
Both pages are visually complete at about 3 seconds. Speed Index measures the shaded area: time spent not looking finished. Smaller is better.
1.09 s Speed Index Good (mobile)
How it’s calculated
Lighthouse records a video of the page loading and computes the visual completeness of each frame compared with the final frame. Speed Index is the area above that progress curve, in milliseconds: the average time at which visible parts of the page are displayed. A page that shows 80% of its content in the first second and finishes at 3 s scores far better than one that sits blank until 2.8 s and then pops in.
How to improve Speed Index
- Everything that helps FCP helps Speed Index: fast TTFB, no render-blocking resources, server-rendered HTML.
- Paint the layout early. Show real text and reserved image boxes rather than a blank screen or a full-page spinner.
- Avoid late-loading web fonts that re-render all visible text.
- Keep heavy above-the-fold carousels and animations out of the critical path.
Should you care about it?
Speed Index isn’t a Core Web Vital, and Google doesn’t use it for ranking. It’s useful as a “feels fast” sanity check, and it’s 10% of the Lighthouse score. Prioritise LCP first; Speed Index usually improves with it.
Check it on your own page
Field and lab numbers for mobile and desktop, in about 30 seconds.
Questions people ask
What is a good Speed Index?
On a mobile Lighthouse run, 3.4 seconds or less is good and over 5.8 seconds is poor. On desktop, about 1.3 seconds earns a score of 0.9.
Is Speed Index a ranking factor?
No. It is a Lighthouse lab metric. Google ranking uses Core Web Vitals from real users.