As many folks probably know, Group Policy slow link detection prior to Windows Vista relied on a series of ICMP pings to determine link speed between the client and domain controller. This process was fairly inprecise and was fraught with issues because many folks have turned off ICMP on their internal networks to prevent malware that leverages this protocol from exploiting this. The end result was that you either had to disable slow link detection, or watch GP processing fail completely.
When Windows Vista and Server 2008 shipped, they introduced a completely new way of detecting slow links for Group Policy processing that no longer leverages ICMP. The process uses the Network Location Awareness (NLA) service to determine the link speed between client and DC, but the explanation of HOW that works has been relatively undocumented…until now. Mike Stephens at Microsoft has written a great blog that describes this process in great detail. Check it out!