Speed Index

Most timings mark one moment: the first paint, the largest paint, the first interaction. Speed Index scores the whole fill: a page that paints a header at half a second and the rest at four seconds scores worse than one that paints everything at two.

• Lighthouse, in Chrome DevTools, in PageSpeed Insights and in the command line tool.
• WebPageTest, which produces the filmstrip the metric is computed from.
• It carries 10% of the Lighthouse performance score in version 10 and later.
• It is not in the Chrome User Experience Report, so it never appears as field data in Search Console.

1. 1

   When the page paints in stages

   A high Speed Index with a good First Contentful Paint means something paints early and the rest arrives late. Look at what fills the viewport last.

2. 2

   When an image or a font is late

   A hero image discovered late, or text that waits for a web font, both show up here before they show up anywhere else.

3. 3

   When comparing two builds

   Run it before and after a change on the same page and the same settings. It is a good relative measure and a poor absolute one.

When the Core Web Vitals are already good in the field, Speed Index is a curiosity. When they are not, it helps explain which part of the load is slow.