Scanning Directories

I recently had to reboot several of my Netware servers. I noticed that two of them took an extremly long time at the Scanning the Directories prompt in the load. The other two loaded fine. What is causing this slowdown. The scan took about 5 mins on1 server and about 3 mins on the other.I run all 4 of the servers seperatley,each server has its own NDS tree due to logistics.
I would appreciate anyones thoughts on this