I play the game at 1920x1080 and my desktop is setup to 3840x2160 . The game launches into 1920x1080, but alt tabbing out and then coming back in, the game is now at 3840x2160 and framerates get crushed.
Going into the graphics settings and manually switching video modes away from and back to 1920x1080 seems to fix it, but this is a bit of a nuisance.