[Richard Berg]

Only whatever anti-malware comes with stock Windows 10.

I took a video with Task Manager running side-by-side so you can see that nothing else is eating system resources: https://photos.app.goo.gl/fBZwszVkrKK2wor58

Examining the video also let me quantify the freeze for the first time: turns out it's 40 seconds, not 20 as I'd previously estimated.  (It may unfreeze faster if I'm not moving the mouse / tapping the screen.)

I've sent time travel captures to Mike so hopefully we'll get to the bottom of it when he gets some free time.