There is a lot of posts on http://forums.iis.net related to network performance. Actually, there was two today!. The problems can be different, but the common thread seems to be network performance. Windows Server 2008 (and R2) enabled a new network feature by default which has been referred to as "Scalable Networking Pak". Some people refer to the feature as TCP Chimney Offload. Either way, in my experience this feature causes more issues than it solves. If you are having a network related issue or performance, this is a easy setting to check and verify if disabling portions or all of them can resolve the issue.
KB article on TCP Chimney Offload
KB Article how to show your settings and disable
I strongly encourage test these changes in a non-production environment before making changes to your production systems!
Microsoft MVP - IIS