Speed Index is the average time page content become visually complete on average. The lower the better.
The Speed Index score was developed when TTFB (Time to First Byte) isn't always the best measure of a site's overall performance.
The Speed Index metric starts by calculating how fast things are rendered at different points in time. The score is calculated through different algorythms. One hiccup can greatly affect this score.
How to lower the score?
Google recommends two specific areas of attack here:
- Optimizing content efficiency: Keeping track of each byte's delivery and delivering them quickly in page loading.