This is a possible solution.
I have been trying to solve this issue for weeks. There have been many threads about the issue but no solutions I could find.
This seem to work for me. Let me recap what I have in place.

Netware 6.5 sp3
Client 4.91
Windows XP Pro sp 2
Iprint Client 4.05 (or 4.06 didn't matter)
Trying to assign iprint printers through a zen policy to the workstation (not user)

My problem was that as soon as I added iprint client my login times would increase. Without iprint client about 15 seconds and with iprint client about 2.5 to 3 min. Too much time for a school and changing of classes. Anyway, through a lot of trial and error I narrowed it down to a Novell Xtier service (used Msconfig.exe to find it) running on the workstation. If I disabled it, workstations flew in login however printers would not come down reliably. But it did narrow down the problem to a Management Agent issue. We installed the Manager Agent that came with Zen 6.5 believing that it was the most up to date (6/16/04). There is a newer Managent Agent (ZfDAgent.msi) that comes with Zen 4.01 interim release 6 (3/7/05). That fixed all my issues and interestingly enough the newer agent does not load the Novell Xtier service anymore.

Good Luck