All I can tell you is that when using IE7 it takes about 10 secs longer for pages to load. And while waiting everything freezes up. No problems with Firefox. Just tried again with IE7 and still slow.
I think the IE7 issues are some javascript speed problems (something with the way IE7 loads javascript). As has been said before, the site hasn't been fully optimized yet, once the bugs in actual functionality have been fixed we will look at ways to remedy this (if nothing else we can just dial down all 'extra' javascript on the page when using IE7).