I hope you do realise that a lot of the sluggishness of KDE can be caused by other programs, etc. What distro are you using? Distributions like RedHat, SuSe and Mandrake are 'famous' for installing and starting up lots of services you don't really need. More 'handson' distros like Debian, Slackware, Gentoo and the like, have less problems with that. Just like with Windows, having a lot of stuff running in the background makes your 'real work' go a lot slower.
Then the next item on the list. A lot of distro's are not highly optimized. This goes for both the kernel and all the other software. With the apple, they could tune all the applications as well as they could, since it would only run on one family of CPUs. In the x86 arena, you have to cater to a whole slew of CPU families. One has SSE, the other doesn't, one has 3DNow, the other doesn't.. etc. Also, there's a difference in general optimisations for the original pentium, the PII, the P!!!, the P4, the AMD CPUs, etc. Each CPU has it's own preferences in that area.
And last but not least, apple only has to cater for a very limited set of graphicscards. On x86, you can have just about anything. Some have better drivers than others, and some distros have more recent drivers or drivers developped by the hardware manufacturers themselves. If you use the standard driver that comes with X, you shouldn't expect wonders in the performance department.
If you give us some clues about your distro, we might be able to give you some clues about how to at least speed it up a little. I am not claiming it can be -as- fast, but I am sure we can improve it at least a litte