I don't know exactly when this started to happen and where to search the problem (so I will start here
) but kwin OpenGL rendering is unusable slow. Not only that the effects are stuttering very often, there is also a quite noticeable delay when rendering normal stuff like the maximize and minimize buttons. When I keep moving the mouse over these buttons (so that the images keeps changing) X cpu usage goes up to ~40% (and as mentioned they change with a visible delay).
When using XRender this is much better (no delay and X cpu usage at ~3-5%). With this only fade and resize effects are slow sometimes.
So where might this problem be located?