I'm having a very similar problem. I only have ~120 torrents open, and maybe 5 or 6 active at any one time, but some of them are quite massive in size too. Lately KTorrent hangs up all the time, it seems like every time a torrent switches from inactive to active, and today I can't even get it to start properly. The IP filter update starts, then freezes, then KTorrent as a whole freezes. I still haven't gotten it to run at all beyond this. No other programs seem to be affected.
I'm using 3.3.4 with KDE4 on Ubuntu 9.10.