After installing the big April Windows 10 Patch (aka Creators Update) I got massive video lag during the end of match score screens, and in the testing grounds.
Interestingly, my FPS was fine according to the in-game (F9) measure. But the game video was actually lagging badly... maybe 1 FPS the a second of normal speed, then another 1FPS stutter.
I rolled back the Windows 10 update and that fixed the issues.