In reality the Web UI was quite complex. IE6 was in the original requirements and was ditched because it was unusable (luckily the customer agreed, were would have been in a really bad position if not). IE7 has performance issues as well, even after many resources thrown at it. I wasn't involved directly, so i can't tell you technical reasons, but i know the WebUI was "state of the art", Ajax, etc. The generated HTML code was not only huge.. it was extreme ;) Also some JS functions were quite slow and adopting the latest ExtJS release also not possible because of not backward compatible changes. All in all, everyone thought in the beginning ExtJS offers probably the most (featurewise). In the end, many are thinking of either going more lowlevel (jquery) or more highlevel (GWT). This will be evaluated.

From my experience with ExtJS and IE, the problem with IE is that it lazily evaluates stuff. Your ExtJS will load lightning fast in IE6, but when you start click on stuff it then runs the JavaScript to generate the HTML. Other browsers don't do this (or their JavaScript engines are fast enough to compensate). This is all a bit fuzzy. Haven't touch it in months, so I could be wrong or things could have changed.