Speed Index measures how quickly the contents of a page are visibly populated. It captures the visual progression of loading and calculates a score based on the visual completeness over time.
How Speed Index works
Speed Index is calculated by taking video frames of the page loading and computing the visual completeness at each frame. A page that renders content quickly throughout loading will have a lower (better) Speed Index.
Speed Index thresholds
- Good: 3.4 seconds or less
- Needs Improvement: Between 3.4 and 5.8 seconds
- Poor: More than 5.8 seconds
Improving Speed Index
- Minimize main thread work
- Reduce JavaScript execution time
- Optimize images and fonts
- Eliminate render-blocking resources
- Reduce server response times
Related Terms
First Contentful Paint (FCP)
A performance metric that measures the time from when the page starts loading to when any part of the page's content is rendered on screen.
Largest Contentful Paint (LCP)
A Core Web Vital that measures how long it takes for the largest content element visible in the viewport to render.
Lighthouse
An open-source automated tool by Google for auditing the quality of web pages, measuring performance, accessibility, SEO, and more.
Time to Interactive (TTI)
A lab metric that measures how long it takes a page to become fully interactive, where the user can reliably interact with the page.