on windows7 (x64) mirc seems to be really laggy when scrolling, my Font is Tahoma 8pt, and scrolling seems to be really sluggish also noticed it using almost %30 cpu, on a quad core :S

Hmm, maybe have a look at the following thread:

https://forums.mirc.com/ubbthreads.php?ubb=showflat&Number=212675&page=0&fpart=1

just read through that, but still have same problem here, did you find a fix for it ?

No, unfortunately not. No idea what's causing it or how to resolve it, sorry.

In fact I got used to it by now... but still it sucks.

What is the manufacturer&model of your CPU?

edit: also, motherboard, RAM (clockspeed and capacity if possible), GPU (if any)...

Intel Core Quad Q9550
motherboard, cant remember exact model but decent gigabyte one
ram 6gb
gpu 9500gt

does any know if its possible to edit the usage of pgup/pgdn keys for mirc, so that they scroll 1 line up per time, like irssi does, this could be a workaround that would work for me :P

That is Ctrl+Page Up/Down in mIRC
See /help key combinations

Just to add my own experience with it... I have noticed just once that the scroll speed got "laggy" when scrolling through heavily colored text (a bunch of testing I was doing for a script that had a lot of colors on each line and filled almost a full maximized window). As soon as I got beyond that, it went back to normal, but every time I went over that area, it got noticeably slow. I've never noticed that happen before, but wanted to put out the info about lots of color changes being a factor.

Specs:
Athlon XP 6000+, 2GB RAM, GeForce 220, Windows 7 Ultimate 32bit

yeah, noticed this too when text is larger/colored, its ALOT more laggy, really strange bug anyway since my old laptop is a crap amd on xp and has no problem with it on 32pt text even lol
if any1 finds a fix for this plz post here, for now i am using irssi, and its ugly!