loke, 
So I just upgraded my test Fedora VM (running on Virtualbox) to version 40. After that, the mouse was incredibly sluggish in Plasma.
Well, wouldn't you know it. It had switched to Wayland by default. Switching back to X11 made it fast again.
loke, So I just upgraded my test Fedora VM (running on Virtualbox) to version 40. After that, the mouse was incredibly sluggish in Plasma.
Well, wouldn't you know it. It had switched to Wayland by default. Switching back to X11 made it fast again.