I think I found the root problem. /usr/bin/startkde (this script is used also for KDM logins) sets MALLOC_CHECK_=3. Argh!? No wonder I had crashes with programs that were previously working. And the system seemed so much slower.
I deleted the offending lines from startkde and restarted KDE. The machine seems to be back to about the same performance as with KDE 4.6 now.
lol @ all the people claiming "4.7 is so much faster here". With MALLOC_CHECK_ set *globally* for *every* application that starts after KDM? Yeaaah, suuuuure. Nice trolling people.