If you've wanted to know more about why high item counts and restricted view requests can impact the performance of your Exchange environment, we've just released some detailed information about the behavior you may see as item counts in your critical path folders grow. Critical path folders include the Calendar, Contacts, Inbox, and Sent Item folders. Restricted views are data views that restrict information based on search criteria that result in views of only a subset of items in a folder. Performance issues related to these situations are frequently related and can become visible to end-users in the form of slow client access and the dreaded RPC dialog-box popping-up. It only takes a few users who have abnormally high item counts in their critical path folders to cause performance issues which are felt throughout your whole Exchange organization. Learn more about the issue in the topic Understanding the Performance Impact of High Item Counts and Restricted Views: http://technet.microsoft.com/en-us/library/cc535025(EXCHG.80).aspx
