I have read numerous posts and articles since KDE SC 4.5 was released last month bemoaning the abilities of graphics drivers, or lack of, in coping with the rendering requirements of KWin. For some reason members of the web press and some users seem to have ignored the advice of the both the KWin developers and the distribution developers. I suspect there are some "issues" with some of the open source drivers however, it's not right or fair to blame the KWin or it's developers.
Martin Gräßlin has an opinion post on his blog that details the checks that KWin performs when it hits issues with 3D and openGL rendering. They seem quite sensible to me.
I run Arch on a Thinkpad with a crumby Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller. I use the standard open source driver and when I upgraded I followed the advice from the Arch developers (posted on the KDE SC 4.5 release announcement) and added the following to my ~/.bash_profile :
Once I had added that and re-enabled "Desktop Effects" in System Settings I have not suffered any problems. I'm not saying it's a magic "cure all" for all driver/card combinations but rather that perhaps following the advice of the developers is the best place to start when you have issues.