I have to fully agree with the origanl poster and artem, that there is an unacceptable performance problem once you use non-antialiased fonts in kde 4.2. I am using a 8500GT based card and have the same sluggish response.
As a software developer I prefer non antialiased fonts in all applications. The micorsoft fonts are designed to look good and are readable at typical sizes (8,9,10 pt). Its only the non or badky hinted fonts, that need the 'hack' of antialising to look tolerable.
This seems to be a typical problem, where one side is blaming the other, in this case Qt/KDE vs. nvidia with the result, that this problem won't get addressed any time soon. Even more so, as all the ( for me useless ) fancy 3D effects and antialiased fonts is going to be the main use case.