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Speed Index

Speed Index measures how quickly the visible part of a page fills in while it loads. Lighthouse records the screen during the test and checks, frame by frame, how complete it looks. A page that shows most of its content early gets a low, good value; a page that stays blank and then appears all at once gets a high one.

Why it matters

Section titled “Why it matters”

First Contentful Paint and Largest Contentful Paint each mark a single moment. Speed Index looks at the whole way from blank to finished. Two pages can have the same LCP, but the one that builds up step by step feels faster than the one that shows nothing for three seconds.

It’s measured only in the lab test, and it’s one of the five metrics in the Performance score.

What influences it

Section titled “What influences it”

Browser: The most common cause of a high Speed Index is a late start. As long as the browser is loading stylesheets and scripts in the head of the page, the screen stays blank, and every blank moment counts fully. Content that a script inserts later, such as sliders, tabs or whole sections built with JavaScript, fills the screen late too.

WordPress: Many themes and page builders fade or slide content in as the page loads. For a visitor, it looks smooth; for Speed Index, the screen isn’t complete until the animation has finished. Large images and web fonts in the first screen take their own time.

Cache: Without a page cache, WordPress builds the page first, and the screen stays blank until the server answers.

Server: A slow or overloaded server delays everything, including the first frame that shows anything.

In Permo

Section titled “In Permo”

Permo shows Speed Index on its PageSpeed page in the lab values for Mobile and Desktop. Google doesn’t attribute estimated savings to Speed Index, so no finding is ever linked to it.

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Sources