Windows Server 2008 Active Directory

Hi

My company have 2  (WIN 2008 64 bit) AD servers running, with one server in staff network and one server in student network. Both AD are able synchronise together. Both staff and student network are physical separated, i.e staff AD 172.16.x.x and student AD is 172.18.x.x

In student network, sometime WINDOWS login very fast and sometime it gonna take 2-3 minutes to get the desktop ready. I did run some wireshark sniffing and found that: some of the PC did actually query to staff network AD (But It block by firewall).

So my question:

1. How to improve the login time? Can I really set student only go for the AD in student network?

Thanks