What Speed Index means and how to improve it
Speed Index scores how quickly the visible part of the page fills in. Under 3.4 s is good. It rewards showing content early rather than all at once at the end.
Step by step, with screenshots: First Contentful Paint and Speed Index: what they measure and what moves them →
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What it is
Speed Index measures how quickly the visible part of a page fills in during loading. Lighthouse records the load frame by frame and calculates how long, on average, the screen looked unfinished; the lower the number, the sooner visitors see a complete-looking page.
In a Lighthouse mobile test, Speed Index is good at 3.4 s or less and poor above 5.8 s. It makes up 10 % of the Lighthouse performance score. It is a lab metric only, so there is no real-user value for it. A page that shows its header, text and hero early scores better than one that stays blank and then appears all at once, even if both finish at the same time.
What a passing site looks like
- Speed Index: passesfail · −10 ptseffort M
Speed Index
Why it matters. Speed Index scores how quickly the visible part of the page fills in. A low number means visitors see a complete page early, even if scripts are still loading behind it.
- Prioritise above-the-fold content: critical CSS inline, hero image preloaded, everything else deferred.
- Avoid client-side rendering for the first screen; server-render it.
Questions people ask
What is Speed Index in Lighthouse?
It is one of the five metrics in the Lighthouse performance score, weighted at 10 %. Lighthouse captures frames of the page loading on a simulated mid-range phone and measures how visually complete each frame is. Speed Index is the average time the page looked incomplete, in seconds, so it rewards pages that paint most of the first screen early.
How do I improve Speed Index?
Get useful content on screen sooner. Reduce server response time, remove render-blocking CSS and scripts, show text while web fonts load, and avoid hiding the page behind a loader or fade-in animation. Anything that improves First Contentful Paint and Largest Contentful Paint usually improves Speed Index too, because they measure the same early rendering.
What is a good Speed Index score?
3.4 s or less in a Lighthouse mobile test is good, 3.4 to 5.8 s needs improvement and above 5.8 s is poor. Desktop tests run on a faster simulated connection and use tighter thresholds, so compare mobile with mobile. A mobile Speed Index of 4 s is not a fault in the tool; it reflects a mid-range phone on a slow connection.