Comment 6 for bug 903097

I encounter a similar behavior with my netbook (32-bit, Intel 945GSE), KDE 4.8.3 (Kubuntu).
At every login, performances are unbearably slow, despite the fact that the settings report the OpenGL mode as active.

To fix the problem I have to switch the compositing type to XRenderer, apply, then set it back to OpenGL. When I do this, I get prompted with the same message Robert reported for this bug.

By testing it with a freshly created user, I can trigger the bug even without having to logout first. I've tested it on the aforementioned netbook:
-log with the default Plasma Netbook mode (max performances, very fluid)
-switch to Plasma Desktop (still max performances, very fluid)
-In System Settings->Desktop Effects->All Effects change any of the plugins already activated (i.e. Desktop Grid) then press Apply (desktop extremely slow, kwin constantly at 100% CPU usage)
-switching back and forth between OpenGL and Xrenderer fixes the problem
-at the next login, the desktop will be very slow again.