Okay, first of all I'm pretty sure this is specifically a issue with Freesync. I bought a new 144Hz monitor
that supports Freesync and didn't have the problem before.
Specifically, the problem is that when I'm playing a game on my main monitor, the Freesync one, and I fullscreen a video
on my other screen, the game begins stuttering badly until I exit the fullscreen video on the other monitor.
Specifically, I was playing WoW, framerate uncapped, Vsync on (tried it with Vsync off too, to be sure), borderless fullscreen (exclusive fullscreen is unavailable).
On the other monitor, I put a video fullscreen, I switch back to WoW, and in about 1 second, I get terrible stuttering. If I stay motionless and look against dark objects,
I can see their brightness flash from dark to brighter and back, like something is terribly wrong.
Also tested in an exclusive fullscreen game. Same results.
GPU: Sapphire Pulse RX 580
CPU: Ryzen 5 1600
OS: Windows 10
Settings info regarding Freesync:
All special features disabled on monitor, only Freesync enabled, also enabled in Crimson settings
No frame limiter, but I never go over or under the Freesync range (45 to 144fps) anyway
Vsync off in games but tried it with VSync on as well.
Tried on both exclusive and borderless fullscreen games, same results.
Monitor 1: MSI Optix MAG241C 144Hz - this is the one I run the game on
Monitor 2: Generic BenQ monitor 60hz - this is the one I run the fullscreen video on
Both are 1920x1080