NTDS is the main source of information on Active Directory traffic to and from Domain Controllers. It contains a ton of interesting looking counters.
Unfortunately, there is not much written that documents AD performance issues and how to use these counters. This TechNet overview article is a good place to start. There are two main objects to monitor:
1. Information related to the NTDS Object:
2. Information related to the Database Object:
The following represents Microsoft’s recommendations when monitoring Domain Controllers:
It will at least help you get acquainted with the basic terminology that AD uses.