Really good news. My vocabulary may to be too limited to tell you how great the latest changes in IE7 RC1 are. Microsoft announced that they have "recently made some great fixes to our engine to improve the garbage collection routine and to reduce unbounded memory growth." and that one "should see noticeable improvements on AJAX sites in the Release Candidate we shipped last week". Yes indeed, we do see tremendeous improvements.
You can test it yourself in this test case, which has been available in qooxdoo for several months to analyze and measure the IE performance problems. Please look at status bar of your browser (maybe you need to enable the status bar in the IE7 security settings). It will display the execution time of each loop while a large number of objects exists. On my machine the IE6 needs ~1400ms, while IE7 needs ~30ms, which is roughly the time Firefox 1.5 needs. Definitely incredible! Bright future for modern web applications.