Calculating Speed Index from Paint Events

WebPageTest General Discussion
1

In the documentation that defines Speed Index (https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index), there is a section called “Visual Progress from Paint Events”. This section of defines an alternative definition of Speed Index, that is described as more lightweight and does not require video capture.

I would like to know: If I have a private WPT instance, is it possible for me to run tests with this alternative definition of Speed Index? If so, what settings/code do I need to change?

I have a couple of secondary questions:

  1. Where in the source code can I find the precise definition of the paint-event based speed index? I would like to know more about what kind of “filtering and weighting” gets used to compute the final score.

  2. Has anyone compared the “consistency” of the original speed index and the paint-event based one? What I mean is: If I do N consecutive runs for a particular browser and website, how similar will my N speed index measurements be?

My apologies if these questions have really obvious answers — I’m quite new to the tools WPT provides.

2

Chrome hasn’t reported accurate paint rectangles for a while (probably over a year - since the move to GPU) so that method no longer works.