Renice'ing it using a tip from here didn't help much in this case. So I went through the AIM preferences and removed all the news and stock ticker options so that the tickers don't show up at all. This brought the average CPU down to around 2% and gave my system a huge speed bump. Little things like typing, scrolling and menus are all a lot snappier now.
[Editor's note: There are also numerous AIM clients out there, including Proteus and Fire which may also take less CPU, although I haven't compared them.]

