The advice provided here seems all correct - get more RAM, use a lightwieght window manager etc. One more thing which may have damaged your KDE experience is the file indexing that KDE does. Your "top" output shows that the dreaded "nepomukservices" are running (two instances?) and while they are not hogging your resources in this particular snapshot they can really slow things down on a system that is low on RAM. Once the initial indexing job is done however, the whole thing gets more manageable and it does become more of background service which does not interfere so much with your work. You can also turn it off it you find it annoying.