Speed Index and filmstrip don't match.

Am I missing something or does the new Speed Index score not match what’s displayed in the filmstrip and visually complete percentages?

This result has a Speed Index of 1.4 seconds to visually render the above the fold experience but then looking at the filmstrip for the median run it’s only 88% visually complete at 1.4s and doesn’t appear complete till around 2.4 seconds. I’m seeing these kinds of differences across a range of tests.

Those are big differences and even though speed index is doing some very clever colour histogram matching I would expect speed index to align with what’s happening in the filmstrips far more closely. Just how fuzzy is the colour histogram threshold for determining the speed index?

It’s just a mis-understanding of what speed index is. The speed index isn’t a point-in-time measurement and you can’t look at that specific point in a filmstrip and visualize it.

The best explanation I have heard is that it is “the average point in time when pixels are rendered to the screen in their final state” but even that’s not great.

The speed index is essentially an opaque number that gives you a sense of how fast large chunks of the screen are filled in and it was explicitly designed to avoid situations where one small piece of content would drag the time out.

Not sure if you looked at the docs on it but they are here - https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index