Glossary
Speed Index
Not when the page finished, but how soon it looked finished.
By Roger Campos · Last updated: September 2026
TL;DR
Speed Index is a lab metric for how quickly the visible part of a page fills in while it loads. Lighthouse records the load as a series of frames and measures how visually complete each one is; the faster it approaches complete, the lower the score. 3.4 seconds or less is good on mobile.
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How it works
How it is calculated
Lighthouse captures the viewport as frames through the load and compares each with the final frame to get a percentage of visual completeness. Speed Index is the area above that curve: the sum over time of how incomplete the viewport still was. Two pages that finish at the same moment score differently if one shows most of its content early and the other stays blank until the end.
| Page A | Page B | |
|---|---|---|
| Visually complete at 1 s | 80% | 0% |
| Visually complete at 2 s | 95% | 20% |
| Visually complete at 3 s | 100% | 100% |
| Speed Index | Lower (better) | Higher (worse) |
Both are finished at 3 seconds; A felt fast and B felt broken. That difference is what Speed Index measures and what a single "load time" hides.
In practice
Thresholds and what moves it
Lighthouse scores mobile Speed Index as good at 3.4 s or less and poor above 5.8 s. It weighs 10% of the Lighthouse 13 performance score. It improves with everything that gets pixels on screen early: a fast server response, no render-blocking CSS or scripts in the head, server-rendered content, and fonts that display a fallback immediately. It is not a Core Web Vital; LCP is the field metric closest to it.
URLpipe
Measuring it with URLpipe
/lighthouse returns speed-index in metrics, in milliseconds, next to First Contentful Paint and LCP. include_audits: "true" adds the diagnostics behind it — what blocked rendering, and what was slow to arrive.
Try it
Speed Index, on a page you choose
Related terms
FAQ
Frequently asked questions
What is a good Speed Index?
Is Speed Index a Core Web Vital?
Why is Speed Index higher than LCP?
See it on your own pages.
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