What you describe is not normal behaviour for the Admin UI, it should be fast to load if it's on your LAN, you might have some other problem causing this. What is the HD setup on this server, any RAID? What do the following produce (run on the Zimbra server):
Code:
host `hostname` <--backticks ot single quotes
dig yourdomain.com mx
dig yourdomain.com any
post the output of those commands.
Do you have an internal DNS server on the LAN? is this server behind a NAT router? Have you disabled Apparmor & the Firewall on the Zimbra server?