Speed Index is a web performance metric that measures how quickly the contents of a web page are visually displayed to the user. It is a key indicator of perceived performance, as it takes into account not just the load time of the page, but also how quickly the content becomes visible and usable to the user.
Speed Index is calculated by capturing a video of the web page loading and analyzing each frame to determine when the above-the-fold content (content visible without scrolling) is fully rendered. The Speed Index is then calculated based on the average time at which each pixel of the viewport is displayed.
A lower Speed Index indicates that the page loads more quickly and is perceived as faster by users. It is a valuable metric for web developers and performance engineers to optimize the loading of their web pages and improve user experience.
Example (Speed Index Calculation): For example, a web page with a Speed Index of 2000 would be perceived as loading faster than a page with a Speed Index of 4000, as the content becomes visible more quickly on the page with the lower index.