Maybe you guys need to test IE7 in slow machines, because the vast majority of users have less powerfull machines than the IE7 developer teams are using. So maybe test it in ones with average ability, and limited resources. If IE7 is fast in those then
it would be fast in a powerfull machine too.
I have tried Firefox and IE7 on a 400Mhz 256MB RAM. Firefox is much better than IE, less scrolling problem, fast switching between Tabs, even Tabs open fast in Firefox.