I’m getting exactly the same issue, and it’s indeed fixed by logging into Plasma 6 + X11, but I can’t seem to disable compositing. I’ve been regularly toggling compositing using either Alt + Shift + F12 or qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.suspend, but the former does nothing, and the latter is no longer recognized as a method:
Error: org.freedesktop.DBus.Error.UnknownMethod
No such method 'suspend' in interface 'org.kde.kwin.Compositing' at object path '/Compositor' (signature '')
Disabling composing seems to be the only way for me to get decent visual frame rate (as opposed to rendered FPS, which is fine) in games. NVIDIA card (MSI RTX 4080), if that’s any surprise. I know there’s no way do disable compositing under Wayland, but is there any way to do so on X11?
edit: Several reboots didn’t fix this, but after an update, the Alt + Shift + F12 shortcut works (the qdbus method is still not available).