The solution I found is “reinstalling the nvidia drivers”, I don’t really understand what’s causing this but as soon as I reinstall them I go back to the regular 120 fps in the editor…
This is still happening, periodically, both on my desktop PC and Laptop and it’s really annoying, the main suspects are Windows update and Unreal itself but it’s really hard to tell (this happens both in 4.27 and 5.1)