I experienced this same problem and it seemed to get better when I
unchecked the following setting:
In Outlook:
1) Tools
2) Options
3) Settings
4) Uncheck the box "Enable the Person Names Smart Tag"

I don't use MSN Messenger, and found that this feature seemed to slow
down my response time when viewing shared calendars or switching
between mailboxes that exist on different mail servers.




Alan Truism wrote:
On Mon, 25 Sep 2006 13:50:21 -0700, wrote:

Turn on Cached-mode in the client. Depending on how big your Exchange
enterprise is, you may be experiencing issues with everyone making RPC
calls at the same time. Using cached mode allows the user to define in
Send/Receive settings how often Outlook is automatically updated
(default is 5 minutes).

Most of our users are already using cached mode; too be honest things seem
a bit snappier when we turn cached mode off.


One important note: BY DEFAULT, when you enable cached mode, Outlook
DOES NOT turn on Automatically update -- you must enable this after
enabling cached mode.


Are you sure about this?
This does not jive with my experience.