Folders with a high item count can impact the performance of Outlook, as this slows the automatic indexing process. Microsoft recommends having no more than 5000 items, and ideally less than 2500 items, in each folder.
In particular, we recommend that Hosted Desktop clients keep items in their critical path folders - "Inbox", "Sent Items", "Deleted Items", "Contacts", "Calendars" - under 1000 items. Outlook profiles on the Hosted Desktop are configured in non-cached mode, and all emails are accessed directly from the Exchange server.
"My mailbox isn't large!"
Performance issues are not related to the size of mailboxes, but the number of items in folders within the mailbox being accessed on the server.
"What are the critical path folders?"
High item counts in the critical path folders affect Outlook performance because the Outlook application itself relies heavily on frequent access to these folders.
Behind the scenes - as the item count rises, the possibility of requested data being physically adjacent on the hard disk is greatly reduced. To account for the additional number of input/output requests, additional time must be allowed.
"How do I reduce the item count?"
Folders with a high item count can be reduced by deleting items, or moving items into a new folder that is NOT nested within the critical path folder (that is, NOT a sub-folder under a critical path folder).
Non-critical path folders, especially folders created by the user, can handle large item counts because Outlook accesses these folders less frequently.
To identify the total number of items in a folder:
- Right-click on the folder.
- Select "Properties".
- Select "Show total number of items".
- Click "OK".
"My item count is below the recommended maximum, but why is Outlook still slow?"
There are some operations that may take a noticeable time - approximately for up to 1 minute. These include new sort orders and selecting folders for viewing the first time in a session.
Third-party applications may also have an impact on performance. These include:
- Outlook add-ins.
- Email archiving.
- Antivirus.
- Mobile device.
- Voice mail.
- Other applications or Outlook add-ins that create additional views or search folders.
Features of Outlook which involve heavy data aggregation may also result in poor performance:
- Shared Calendars.
- Custom search.
- Filtered views. For example, showing only unread items.
- Calendars with recurring appointments.
- Views consisting of custom data.
"Why 1000 items?"
While 1000 is our recommended maximum item count for critical path folders on the Hosted Desktop, please note that this figure is not hard-coded. As item count rises, there is a progressive degradation in performance.
Further reading
Comments