found a pretty reliable way to make it perform shitty - alt-tab out of the game, do something memory-hoggy, like firefox with plenty of tabs and alt-tab back. it’s often (but not always) super shitty then.
I’m on F40 Plasma, didn’t have that degradation on F39 Gnome when alt-tabbing; but then I had Gnome session crashes frequently, which never happen with Plasma 6.