UX Metrics for the Modern Web
The "speed index" is a metric peculiar to WebPageTest, a popular piece of testing software. Loosely speaking, the speed index is the average amount of time for visual components to be painted on the screen. Pages with a faster "start render" and a faster "visually complete" would have a greater percentage of the screen painted at any time—so the area above the curve would be less, and the speed index would be less (lower is better). "Speed index," however, faces the same pitfalls as "visually complete" and "document complete," in that it is not a good way to measure the load times of non-visual components, which might still be critical to the user experience.
Real User Monitoring (RUM)