Recently we had an perceived performance issue with our Messaging environment. Ultimately it came down to high DiskIO and a large number of people with high Item Count in top level folders.

This is a collection of tips that can help improve/mitigate MS Outlook’s performance with an Exchange server. Complete Outlook/Exchange performance relies on a lot more.

Disable OS client level anti-virus scanning of the mailbox

1. On the Tools menu in Outlook, click Options.
2. Click the Other tab, and then click the Advanced Options button.
3. Click the Add-In Manager button.
4. Click to clear the check box for Exchange Scan and note any other third-party add-ins that may be selected.
5. Click OK three times.
6. Restart Outlook.

http://support.microsoft.com/kb/839862/en-us
Client level anti-virus scanning add-in and other third party add-ins can have an impact on the Exchange server and Outlook client performance. This assumes you have Anti-virus scanning on the Server or messaging gateways somewhere. Google Desktop also falls under this category.

Disable remove extra line breaks in plain text messages

1. On the Tools menu, click Options
2. On the Preferences tab, click E-mail Options
3. Click to clear the remove extra line breaks in plain text messages check box.
4. Click OK twice
5. Restart Outlook

http://support.microsoft.com/kb/308319/en-us
When you open a large plain text message in Microsoft Outlook, it may take a very long time to open and Outlook may appear to stop responding (hang), and your CPU utilization may go to 100 percent.

Disable Instant Messaging integration in Outlook

1. On the Tools menu, click Options, and then click the Other tab
2. Under Person Names, uncheck clear the Enable Person Names Smart Tag check box and the Display Messenger Status in the From field check box
3. Click OK

http://support.microsoft.com/kb/839862/en-us

Check if Planner Options feature is enabled

1. On the Tools menu, click Options.
2. On the Preferences tab, click Calendar Options.
3. In the Calendar Options dialog box, click Planner Options.
4. In the Meeting Planner section, clear the Show pop up calendar details check box and the Show calendar details in grid check box.
5. Click OK three times.

Planner Options is a feature that was originally introduced in Outlook 2002. Planner Options gathers much more data than earlier versions of Outlook if the Scheduling tab is clicked when a user creates, updates, or reads about a meeting. If disabling this feature resolves the excessive RPC dialog boxes, you must determine if disabling this feature works for your business. If the Planner Options feature is disabled, the user can see the attendee's free/busy information, but additional details are not viewable.
http://support.microsoft.com/kb/839862/en-us

Check if Word is the editor

1. On the Tools menu, click Options.
2. Click the Mail Format tab
3. Note if the following items are selected
• Use Microsoft Office Word 2003 to edit e-mail messages
• Use Microsoft Office Word 2003 to read Rich Text e-mail messages

Disable Word as the email editor or note down settings and live with occasional pauses.
http://support.microsoft.com/kb/278214/en-us

Reduce the number of items in a folder.

The numbers vary depending on your version on Outlook. The most common old number seen is keep a given folder count under 2000 items. The referenced TechNet article indicates 3,500-5,000 for Outlook 2003 on Exchange 2003. If someone is using custom Views on folders then the number of items in the folder will be lower as custom views has a performance impact on Outlook retrieving message. Past experience and ‘off the cuff’ chat with some MS Support folks suggest a recommendation to limit item count in folders to approx 2000. This item count recommendation also applies to the Calendar folder as well.
http://support.microsoft.com/?id=905803

Outlook 2003 Cached mode

Whether to enable or disable Outlook 2003 cached mode depends on how a person uses their Outlook client. If someone has no additional mailboxes mapped (is not a delegate) then they are a good candidate for using cached mode. Because cached mode Outlook is not connected full time to the Exchange server, scheduling meetings can result in slight delays while Outlooks meeting planner connects to the Exchange server to lookup free/busy information.

So, if someone access another mailbox regularly or open another users calendar regularly, then Outlook cached mode should not be enabled for them. Note: I used cached mode even though I have additional mailboxes connected because I don't use the other mailboxes that often.
http://support.microsoft.com/kb/870926

Client side rules

Client side rules have a client impact on Outlook. Generally these are rules that move mail from the mailbox to a PST. See what rules can be eliminated/consolidated or changed to become server side rules. Add “Stop processing more rules” to existing server side rules to prevent excessive rules processing
http://support.microsoft.com/kb/870926

Remove PST files off network shares

Personal folders are unsupported by Microsoft over LAN and WAN links. The larger the PST file gets the more resources it takes to access. This can cause Outlook to freeze just like when PST files get corrupt. Also, accessing PST files over a WAN link through VPN can make the effect worse. The WAN link already has more latency in general than a LAN connection does and if you toss in the latency added by VPN and often by any additional wireless connection common for a home or cafe connection, PST files over 200 MB in size can grind your Outlook to a halt. Your experience may, of course, vary.
http://support.microsoft.com/kb/297019/