I had been getting the "A script on this page is causing Internet Explorer to run slowly" error in IE7. In my case, it only occurred after flushing the cache, when the Admin Menu was built and rendered as part of the page's HTML.
I was able to fix it with a patch to the jQuery selector that colorbox uses. It was this:
$('a, area, input', context)
I changed it to this (which, I think, is equivalent):
$('a.colorbox, area.colorbox, input.colorbox', context)
I think the second one is just more efficient (it doesn't need to filter on every link, etc.).