Some details first:
Windows XP Workstations, in an AD domain, with MSGina first in the chain, novell client (4.91 SP4) using passive login.
With out the ZCM agent (and on our old old old Zen4 agent), logins are fine, no slowness or anything.

Install the agent, and once you start up, press ctrl+alt+del and it takes about a minute or more to show the login dialog. Then once you type in credentials, it takes another 2 minutes or more with just the background displayed, then does setting up computer and processing zcm login. Once logged in everything seems fine.

I've tried tons of stuff, from XP/Domain based policy things that usually resolve slow AD logins, but once I uninstall the ZCM agent, all is fine. It also does not happen to every type of device, which makes it even more frustrating. I'm thinking of just waiting for SP2 and praying that fixes the slowness, but I don't want to put all my cards in that deck.