I've recently encounter problems with slow domain user login. It takes almost 30minutes to login.
The weird part is for the machine which is on vlan 172.16.0.0, the user was able to login as normal.
For the machine on the 10.80.1.0 subnet the user takes around 30 minutes to login.
If the machine was change to 172.16.0.0 subnets it was able to login as normal.
I clear the dns cache on the dns server and run dcdiag to check on both dc controller.
Could it be a network problem?