When using WebPageTest Scripting on the Dulles IE11 location, the filmstrip is displaying ~5s+ of a seemingly Windows-related update popup (the blue one in the filmstrip below). This popup interferes with the Speed Index calculation, returning false results.
You can see two instances of this happening here:
It looks like the Windows runner for this particular runner has an update pending that it cannot download.
Let me know if I can be of any help!