Two nights ago I reverted to KDE 3.5.1, as the UI was getting less and less responsive.
Krunner, Kate, Kdesvn for some reason felt slower than when I first installed KDE 4.1. There was also a silly issue with Kate (my primary text editor) which didn’t help.
Maybe it was the hardware, as my laptop was bought in Aug 2006, I’m not sure. But now I know why Lenny was released with KDE 3.x.y, it just feels more… complete.