Take a look at Google’s documentation on their Speed Index. What it’s measuring is how quickly the page contents are visually populated. The main difference between Speed Index and the regular old Page Speed is this:
Speed Index measures how quickly stuff appears up on the page. Page Speed measures how long everything takes to 100% load.
This is a much better way at determining how speed impacts user experience.
Google does a lot of the development for WebPagetest as part of our efforts to make the web faster (WebPagetest is an open source project I started before coming over to Google).
The Speed Index is a new metric available in WebPagetest for evaluating the end-user performance (and comparing pages against each other) and it is something we developed at Google but don't go reading too much into that.
It's helpful for comparing user experiences and was developed specifically for WebPagetest.