On the heals of my IE 7 review I took a closer look at the performance issues in IE and it looks like the performance issue I was seeing is heavily tied to the new native PNG support. One of the projects that we (at Raizlabs) are working is a fairly complex AJAX application that makes fairly heavy use of transparencies. To support both IE and Firefox we've been using conditional comments with an ActiveX control to render PNG's on both IE and Firefox.
We had previously assumed that when IE7 was released the native support would allow us to use the native support but it looks like the ActiveX trick renders significantly faster then the new native support. It'll be interesting to see if IE7 ships with this issue.