The Speed Index metric is the most basic and fundamental measurement of the performance of a website’s user experience.
Google introduced this metric to help site owners track their mobile pages by using a star rating system: 5 stars for “above average,” 4 for “good,” 3 for “OK,” 2 stars for “underperforming” and 1 star for “minimal.”
The good thing is that there are a range of other metrics you can explore, like the time it takes to load an interactive page or if any errors occur in your website.
This metric measures how fast a mobile page can handle input events from a user. When testing the speed of a webpage, Google uses 3 metrics: Start Render, Time To Interactive and Speed Index.
Start Render is when the page content becomes visible after request. Time To Interactive is when the page is able to handle user events like scrolling, taps, etc.
And the Speed Index metric measures how fast a mobile page can handle input events from a user.
To find this metric in Lighthouse, head over to Performance -> Speed Index on desktop or under the Tab overview section on mobile. On this screen, you will see 2 charts: one for Start Render and one for Speed Index.