I've noticed since patch 5.0.4 that scrolling through the achievement frame when there are many achievements listed, in particular the 'Lich King Raid' one (which is honestly a mess), seems to take up a lot of CPU as it often causes temporary freezes when scrolling quickly.
I've asked a few people but it doesn't seem to happen to them. I've got a decent processor though (Intel core i7 3.4GHZ) so that can't be a problem. I've turned off my addons too because one thing does seem to slow it down a tad more, namely hooking AchievementButton_DisplayAchievement, but it still happens without.
Yes, it happens to me with anything involving scroll frames. I believe its related to the use of unthrottled OnUpdate scripts. It's severity seems proportional to the complexity of the frame elements being redrawn.
BTW, I have an overclocked (3.8 GHz) Intel Core i7-875K processor and an NVIDIA GeForce GTX 560 Ti graphics card.
On my windows 7 PC with a 2Quad Q9450 @ 2.66GHz it takes 38% while standing around in OG but its not going higher while scrolling in LK archives but i get a small lagg and fps drops from 75 to 14 my graphics is a ati hd 5870 (1gb)
I also tested on my macbook (end 2011) i think it has a 1gb graphics and a i7 2.2ghz and it works fine I don't get any fps drops it stays 30
I have 8gb ram and a SSD on both.
Testet with and without addons + and "ultra" wow options on both.