The Speed Index, or SI, is one of six metrics that make up the Performance section of a Lighthouse report. Focused primarily on the speed and performance of a website, the Speed Index is used to help measure how quickly page content is displayed to users, specifically above-the-fold content, which is a key factor for the user experience. We have already mentioned some of the metrics already, but to reiterate, the other five metrics that make up the Performance section in a Lighthouse report include First Contentful Paint, Time to Interactive, Total Blocking Time, Largest Contentful Paint, and Cumulative Layout Shift.
What is the Speed Index?
How the Speed Index is Calculated
The Speed Index calculated by reviewing the frame-by-frame page load process and measures the time it takes for your page to be visually complete. From there, it then compares your web page against other web pages to calculate the final score. A Speed Index score of 3.4 seconds or less is considered good and will show up in green on the Lighthouse report. A time of 5.8 seconds or more is considered slow and should be addressed.
How to Improve the Speed Index Score
The Speed Index closely tied to other Lighthouse metrics , like First Contentful Paint and Largest Contentful Paint, so it is one of the indicators that you will want to pay attention to when reviewing this and other indicators in the Lighthouse report. However, in terms of the overall Lighthouse Performance score, it makes up only 10 percent of the score, which is low compared to the other Lighthouse Performance metrics. And unlike other Lighthouse metrics, there is not a lot of suggested actions provided to just improve your Speed Index, since it tied to the performance of multiple metrics, so you will need to utilize the Lighthouse optimization suggestions for other metrics to improve the Speed Index score.
Lighthouse is an open-source tool that is used to run an audit against your web pages and provides scores and suggestions for improving page performance, accessibility, SEO, and more.
PageSpeed Insights is a tool created by Google that reports on the performance of a page on both mobile and desktop devices, and provides suggestions on how that page may be improved.