I was patient. I truly was. I tried and tried. But after an intense two-days session with KDE 4.1.2 I've lost my patience, once and for all.
The fact is: KDE 4 is still (i) slow, (ii) utterly unusable, and (iii) prone to crashes like nothing I've seen since Windows 3.1. And I'm afraid that it will stay that way for a very long time.
The problem for a long-time and happy KDE user like me is: there's no official milestone for the further development of KDE 3.5. Furthermore, several applications already exist only in a KDE 4 version, such as, for example, krusader or yakuake—and believe me, the limitations show.
What to do, then? I really don't know. It saddens me that KDE is ruined in that way, and I'm thinking about alternatives. Gnome isn't one, though. Perhaps xfce? Just having to think about that angers me.
It turned out that the NVidia driver was responsible for this phenomenon. :/ See here. With the open driver (nv) KDE4 is just as fast as KDE3 or perhaps even faster.
I could list dozens of examples, but just look at the ways you're required to go to add programs (as opposed to widgets) to the panel and to move them. If there's none of the frequent plasma crashes, ask yourself: now, was that convenient? Or even intuitive?
I've tried it on more than one distribution.