>>409968929
You're running out of vram, drops you from smooth whatever to a handful of frames per second no matter what you're doing.
With the mere 8gb a 2080s has it's going to be a very common occurrence.
Opening the steam overlay lowers the game resolution to half, thus greatly reducing vram usage, temporarily "fixing" the issue.
It will often also "cause" the issue, because other vram starved processes will jump at the opportunity to eat some more, so when you exit the overlay there won't be enough to satisfy it and you get your stutterfest.