Outlook 2003 slow over VPN

I have a client who has Exchange 2000. He uses a DSL line at his busines and his home. At night he takes his laptop home and connects to his exchange mailbox over a VPN connection. His laptop has Outlook 2003 and is setup in Cached Mode.

Here is the problem. While at home he can open Outlook fine. The first time he trys to create a new message though Outlook hangs anywhere from 3-5 minutes. His mailbox size is over 400 Megs and he uses 3 sets of archive folders.

What could be causing this initial delay when creating a new mail message and what can I do to alleviate it?