I've been experiencing the same problem. mIRC worked fine on my old PC on XP, but now that I switched to Vista 64bit, this started happening. Is it possible its due to many networks and channels being open? On 59 channels over 11 networks at the moment and I noticed when I start up mIRC that at first its fast, then after I join ~15th channel or so it starts lagging. It's really annoying and I've no idea really what the problem could be (yes, I tried turning off firewall, closing background processes, fresh install etc.).
