Some days ago, after I had played around with some plasma widgets, I recognized Medea (AMD Athlon 2000+) becoming very slow and not very responsive after about 20 minutes of operation. Good old console "top" yielded that plasma was eating up CPU time up to 80% and was consuming a lot of memory. Short googleing didn't find any really helpful results, although the problem seems to appear at least for some users.
I was too lazy to sort out what was causing the problem, but deactivating all widgets I had started (calendar, clock and the analogon to knotes) solved the problem and my computer runs nicely again.
For the interested guys: openSuSE 11.1, KDE 4.2