How many rooms are you in?
Past experience with reports like this has shown that the people that are having this problem are in, imo, way too many rooms for chatting purposes.
Insted of give me a stupid answer you wuld do like RoCk did... I have set it to so littel as i can do. 200, Insted of my mIRC taking 200MB ram it take 25MB ram so it helped me alot
You can change the window buffer size by going to
options (alt+o) > other > window buffer
Will just say thx and it sloved many of my problems

Thx Again :=)