Microsoft Outlook 2007
There are a number of known performance issues with Outlook 2007. Test the server environment with Outlook 2003 with the latest Service Pack to determine if the performance issue is application specific. A number of performance hotfixes have been released for Outlook 2007:

Description of the update for Outlook 2007: April 13, 2007
http://support.microsoft.com/kb/933493
This update fixes the issue that is described in the following Microsoft Knowledge Base article:
932086 (http://support.microsoft.com/kb/932086/) You may experience performance problems when you are working with items in a large .pst file or in a large .ost file in Outlook 2007
NOTE: This hotfix appears to improve Outlook 2007 performance when using PST/archive files

Description of the 2007 Office hotfix package: February 26, 2008
http://support.microsoft.com/kb/948950
This hotfix fixes the following issues that were not previously documented in a Microsoft Knowledge Base article:
- A server that is running Windows Server 2003 experiences high CPU usage, and Microsoft Office Outlook 2007 runs very slowly.
This issue occurs when many users run Outlook 2007 in Terminal Server client sessions that are connected to the Windows Server 2003-based server.
NOTE: This hotfix appears detrimental to performance despite the comments in the article.

The use of Personal Folders or Archives (.PST files) is unsupported and causes performance issues:

Personal folder files are unsupported over a LAN or over a WAN link
http://support.microsoft.com/kb/297019