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This issue is actually better posed on the IE forums, as we only provide support for Exchange Online and the other Office 365 services here. While I'd suggest that you check to make sure that the machine has enough hard drive space and virtual memory, this is something you'll want to post on the IE forums as it's a general performance issue related to Internet Explorer, rather than Exchange Online.
Did you ever get a fix for this? We are having the same problem and it ONLY HAPPENS WHILE TRYING TO OPEN EMAILS ON OFFICE 365. It may very well be an issues with IE, but it is funny how no other website, including HOTMAIL, GMAIL and GOOGLE APPS does not have this problem. I have disabled all add-on's tried using In-private and nothing works. There are some other things I have yet to try, but if someone else found a fix already, it would save me some time searching and troubleshooting.