Well i wanted new theme in my Kubuntu 11.04 with KDE 4.6, and i have noticed that when i change my theme from Oxygen to other in System Settings my system gets slower. Why this happens, i can see the notice when i use wobbly windows and an other theme than Oxygen and when moving window it is so laggy, but with default Oxygen theme there is no lag. Is it possible to fix it, only one theme downloaded from that thing works without lag, but i want use other themes too. I use an ATI Mobility Radeon HD 4250 as my graphics card and i have installed drivers for it from Kubuntu built-in Additional Drivers program. Can that help if i remove those ATI drivers from Additional Drivers program and download my drivers from ATI site and install them? Or is my computer too bad for those themes? My specs:
CPU: AMD Athlon II X2 Dual-Core 2.1 GHz
RAM: 3 GB DDR3
OS: Kubuntu 11.04
GPU: ATI Mobility Radeon HD 4250
Hard Drive: 320 GB 7200 RPM.

And my computer is an laptop.

Which theme are using changing? The Plasma theme? The application colours? The window style? The application style?