One thing you haven't specified is if you're running any scripts. You mentioned mIRC getting sluggish with the release of version 6, which is of course when multi-server support was added. Multi-server support is one of the things that is most likely to make inefficient or excessive code slow mIRC down noticeably in the way that you've mentioned. If you are using scripts you should try unloading them and restarting, then see if mIRC's performance is any better. If that is the problem then you should either look for a new script or try and fix the one you've got by streamlining code - especially any code related to events such as on INPUT, on NOTICE, and on TEXT.
I run mIRC on a couple of machines (usually a P500, 384MB RAM) and I never experience any sluggishness, at least none that I can't easily explain by way of other apps using resources heavily.
