Why does IE7 run slow against a .NET 3.5 project on Windows Server 2008?

We have a web application (built on vb.net 3.5, heavy use of JS for navigation, page validation, UI interaction) that runs very well on many of the new browsers, however, some of our clients (for reasons outside of our control) use IE7 and complain constantly of extreme slowness that you don't see when you load the software up in a more recent browser (FF, Chrome, Opera or IE8 for example), even on the same computer. We have ruled out network issues, viruses, and hardware. Some time ago we heard that there may be an issue where IE7 works slowly when asked to render a site that is served from a MS Win2008 Web Server, has anyone ever heard of this and could they possibly share what they have so that I can explain it to our client.