Speed Index (SI) is a page load performance metric measuring how quickly the contents of a page are visibly populated. Speed Index is dependent on size of the viewport and expressed in milliseconds: the lower amount of time the better the score.
Speed Index was introduced to address issues with other milestones and metrics and provide a gauge of real user experience. Speed Index has been implemented in several common audits including WebPageTest and Lighthouse.
Speed Index is calculated by what percent of the page is visually complete at every 100ms interval until the page is visually complete. The overall score is a sum of the individual 10 times per second intervals of the percent of the screen that is not visually complete.
Diagram showing how above the fold content can load before the page load event and is measured by Speed Index: