Are you using Nvidia? I remember there was an issue with Nvidia and Aurorae (one of the window decorator used by kde). Try changing system theme in kde settings in case one of them doesn't trigger the slowdown issue.
Same issues on both my nvidia desktop, my amd desktop, and my amd laptop.
I definitely first suspected nvidia, since well, it's nvidia. But I wouldn't have the exact same issues on other machines without nvidia as well then..i hope