Abnormally low FPS unless I enable AMD performance overlay

Recently I started having a weird problem. In games, even not demanding ones, I get this very low fps. Yet, when I enable the AMD fps overlay it comes back to normal. I think it has something to do with fullscreen but I can't really pinpoint it because I get it in exclusive fullscreen and borderless fullscreen both.

Might be useful: I am usually running EarTrumpet and ShareX (screenshot app) in the background (tray).

It is really weird, cos I have never experienced something similar in my entire gaming life.