This was one issue I worked on for quite some time a few months ago. We found that Vista clients were taking forever to download group policies from domain controllers in the regions while Vista SP1 and XP clients did not see the issue. If the local DC was running LH or Windows 2003 SP1, we didn't see any issue. If the server was running Windows 2003 SP2 though and had a certain type of hardware we would see the latency.
Based on customer reports, Windows Dev learned of more widespread devices in the network that don’t understand greater window scale factors. So, they improved the heuristics in SP1 for when to limit or disable auto-tuning, which is why we only saw in in Vista RTM and not SP1.
The bug is in Windows 2003 SP2 and the details can be found here: http://support.microsoft.com/kb/947773
So if you find that you have vista clients experiencing network latency when hitting a Windows 2003 SP2 server (File server, DC, Exchange, etc) then you should look into this hotfix.