Speed Index (SI) is a page load performance metric that shows you how quickly the contents of a page are visibly populated. It is the average time at which visible parts of the page are displayed. Expressed in milliseconds, and dependent on size of the view port, the lower the score, the better.
The calculation calculates what percent of the page is visually complete at every 100ms interval until the page is visually complete. The overall score, the above the fold metric, is a sum of the individual 10 times per second intervals of the percent of the screen that is not-visually complete.