If you are having a poor experience, and you are unable to meet the system requirements of KDE SC 4.7, then the easiest interim solution might be to change the desktop rendering backend to Xrender.System Requirements
In order to get the most out of these releases, we strongly recommend to use the latest version of Qt, as of today 4.7.4. This is necessary in order to assure a stable experience, as some improvements made to KDE software have actually been done in the underlying Qt framework.
In order to make full use of the capabilities of KDE's software, we also recommend to use the latest graphics drivers for your system, as this can improve the user experience substantially, both in optional functionality, and in overall performance and stability.
For other readers, for some balance ... KDE SC 4.7 is a fantastic experience for some users:
Thanks for driving my poor GeForce 6150 towards graphics nirvana. Blur works amazingly here, effects are smooth and I can still see my graphics chip throttling down to 100 MHz.
Thanks also for enabling me to use the Lanczos shader with my GeForce 8500 GT. I never saw that feature running so smoothly in a computer.
Now, with KDE 4.7, I can pump more effects, get more performance and, in some cases, use even less CPU than never before. Thank you, KWin performance in KDE 4.7 is, for the first time, GREAT.