Gentoo explains the debug USE-Flag like that:

Tells configure and the makefiles to build for debugging. Effects vary across packages, but generally it will at least add -g to CFLAGS. Remember to set FEATURES=nostrip too

Well, for KDE-packages that mainly means: Dump everything to .xsesssion-errors. I realized this being the culprit of my overblown .xsession-errors about a month ago - but today this file grew by 4GB in about 3 hours and that surely was too much for me. Reason for that usually is KMail, which is a good mail-client, but surely sucks internally. Having kio_imap4-processess hanging around, doing nothing else but eating 100% of processor-time.

Suggestion to everyone: At least build KDE-packages without debug-flag.
It's just sad, that you can't call the programs with --debug or something like that - it has to be compiled with it. :(