Borderless/Fullscreen Weirdness in KCD

[JimTheBeet]

I recently picked Kingdom Come: Deliverence back up after the trailer for the sequel came out and have been monkeying with performance. Everything I see online says that performance should get better in fullscreen mode, and that it should also be able to fix terribly low cutscene framerates. For some reason, though, I get far better performance in borderless mode (difference of 40 fps in Rattay to 90-110). The drawback, though, is that every fix I see for terribly stuttery 15 fps cutscenes seems to rely on full screen mode! Does anyone have any idea what the heck could be going on with this? Or, more broadly, why game performance often changes between windowed/borderless/fullscreen options? It goes without saying that KCD is a rather unoptimized mess, but other than that I'm at a loss for an explanation.