Speed Index is a metric originally introduced by WebPageTest that aims to measure how quickly a web page's content is visually displayed during load. It uses a video of the loading progress, just like the one you can see in PageVitals, and it compares the visual progression between frames.
How is Speed Index calculated?
Speed Index is calculated by simulating how a web page loads in a real browser. It uses a video of the loading progress, just like the one you can see in PageVitals, and it compares the visual progression between frames. The method used to calculate Speed Index is based on a set of frames that capture the visual changes on the page as it loads. The frames are taken at regular intervals during the loading process, and the differences between them are used to calculate the score. The lower the score, the better the perceived performance of the website.
What is a good Speed Index value?
A good Speed Index value is generally considered to be under 3000 milliseconds (ms). However, the optimal value can vary depending on the type of website and the content it contains. A website that contains a lot of images or video may have a higher Speed Index value than a website with mostly text content.