KDE4 has worse graphical performance than KDE3, that is know.
It is because bad drivers...

But what I'm experiencing is general bad responsiveness (a lot disk activity, like if swapping is going on). After some usage everything slows down to a crawl!

I checked free output, but swap is untouched:
Code:
free -m
             total       used       free     shared    buffers     cached
Mem:          1519       1487         31          0          3       1108
-/+ buffers/cache:        375       1144
Swap:         1027          0       1027
My machine is:
Athlon 64 2800+
1.5 GB ram
~500GB of disks
Nvidia 7600GT or Radeon 9250

For e.g. opening dolphin → disk activity raises a lot
opening ktorrent → disk activity, opening opera → disk activity.
Cold starts are long, but warm are nothing shorter. Under KDE3 there isn't that much disk activity. What could be the reason in first place for that?

Opening a movie from ktorrent → it needs seconds before it starts to play!
And even worse, closing mplayer will lead to a black screen and disk activity will raise again (really seems like it is swapping, but free shows nothing).

In KDE3 movie opens in under a second, without disk trashing around that much and closing it it's like nothing ever happened, desktop is super responsive.

What could be the problem of that?
I like a lot KDE4 but this makes me really think about using it ever again.

KDE4 was installed from Factory repo (4.2 RC).

I also retested everything with a radeon, just to be sure nvidia drivers aren't interfering.