MSN Explorer needs to sync your email. So, working in MSN Explorer might be affected if your mail is synchronizing. Once it is in sync, browsing the web should be fine. Note that you don't need MSN Explorer and you can cancel your MSN service, while still keeping your @msn email address, which can then be accessed via http://www.hotmail.com or with the email program of your choosing. Call customer service to cancel if you wish.
If you wish to use MSN Explorer, you need to keep the service. The following may then apply to your situation:
How many messages are in your Inbox and other folders?
Does everything else work normally as far as performance is concerned?
This is probably due to the fact that you have not cleaned up your email and MSN Explorer is attempting to synchronize all data each time it loads. I suggest deleting messages you no longer want - including from the Sent folder, moving messages to folders other than the Inbox that you wish to save - consider moving them to folders "on my computer" so they no longer sync, and emptying the Deleted items/trash folder. This should improve performance significantly.
If you move items to folders on the local PC, be sure to backup your MSN mail periodically as a drive failure on the PC will cause you to lose these messages.
Backing up email: http://answers.msn.com/solution.aspx?solutionid=d830f08c-414d-49ae-91cb-df58aa44832f
-steve