The major focus on Internet Explorer 7 has been security and particularly the phishing filter.
In a recent article, Microsoft has stated:
When you use Windows Internet Explorer 7 to visit a Web page, the computer may respond very slowly as the Phishing Filter evaluates Web page contents.
The reason is this happens is if you have too many pages open or the website you are browsing has too many frames. Since IE tries to test all the pages, it can result in high CPU Usage and slowing down of your computer.
The update has not been released on Patch Tuesday, but has been done in a individual update.
Update for Windows XP with Service Pack 2, x86-based versions
(WGA Validation required)