Just a couple of notes...
mIRC is the best IRC client for windows, if not becouse of his look then it's couse you can write your own scripts and change the way mIRC looks and things that it does...
One more thing, I have tried many irc clients for linux, and I had no problems with a large number of users. So I think it has nothing to do with processor, but it's posible that your connection (if you have a small one) is the problem.
