The first step towards finding the cause would be to figure out if the slowing down is in the browser or the server. From what you've said, I'd be inclined to think it's the browser (especially since the browser is IE )
Does this problem only occur when a user is logged in to the CMS as a user, or does this problem happen even if the user is just loading through page after page without being logged in? Is the problem always fixed by closing the browser, or does this slowing down sometimes persist even after the browser has been restarted?
If the problem is always fixed by restarting the browser and it happens regardless of whether or not the user is logged in, then it's almost certainly a problem with the browser.