KDE 4.2 is great, I love it.

While the KWin manager runs slowly, so animations are not quite smooth as compiz or Metacity.

I have tested FPS with glxgears:
Kwin:
14944 frames in 5.0 seconds = 2988.758 FPS
15817 frames in 5.0 seconds = 3163.388 FPS
15851 frames in 5.0 seconds = 3169.792 FPS
15798 frames in 5.0 seconds = 3157.764 FPS
15541 frames in 5.0 seconds = 3108.100 FPS

Compiz in KDE:
20800 frames in 5.0 seconds = 4159.993 FPS
21238 frames in 5.0 seconds = 4247.556 FPS
21325 frames in 5.0 seconds = 4264.955 FPS
20489 frames in 5.0 seconds = 4097.161 FPS
20762 frames in 5.0 seconds = 4152.393 FPS

Metacity:
27813 frames in 5.0 seconds = 5562.421 FPS
28107 frames in 5.0 seconds = 5621.280 FPS
28101 frames in 5.0 seconds = 5620.062 FPS
28097 frames in 5.0 seconds = 5619.255 FPS
28109 frames in 5.0 seconds = 5621.698 FPS

Geforce 7600Go with 180.27 nvidia driver Fedora9

Try switching between OpenGL and XRender 3D effects to see if it has any effect.

Metacity doesn't use OpenGL for compositing by default so that's why you get a higher glxgears framerate, it's not a fair comparison to either Compiz or KWin.

No idea why Compiz is so different to KWin though, will have to look into that.