Important: Some of the information below asks you to make changes to your software and/or usage of them. We recommend that you discuss these changes with your local IT administrator prior to making these changes.

• For best support and access to latest features of Office/Outlook, please make sure you are using most up-to-date version of Office/Outlook. See Microsoft 365 - Download/Install Microsoft 365 article for more details.

• Outlook add-ons can be the cause of significant performance degradation. For example, the iCloud add-on for Outlook for Windows has had many issues since its release and should be disabled if it is not needed. It is recommended that any add-ons not needed by the user be disabled. Review this resource to manage your add-ons.

• Outlook for Windows uses graphics acceleration to improve visual performance. However, this feature is not compatible with all graphics cards and can actually degrade performance, cause blurry text, and cause Outlook to hang. Graphics acceleration can be disabled by going to File | Options | Advanced and checking the “Disable Graphics Acceleration” box.

• Limiting the number of calendars in the “My Calendars” folder will improve Outlook performance. These calendars are synchronized regularly while Outlook is running. If a shared calendar is not heavily used by a user, it should be moved (you can drag and drop it in Outlook) to the “Shared Calendars” folder.

• Microsoft recommends enabling cached mode in Outlook clients connected to Exchange Online. This will improve performance and connectivity issues for the average user. However, administrative assistants that have a critical business need to receive shared calendar updates in real time may be adversely impacted by this setting and can have cached mode turned off if affected by calendar update latency.

• Check the connection status for Outlook. This can be done by holding Ctrl key and right-clicking the Outlook quick-launch icon in the system tray near the clock. Select connection status. Here you will see the number of persistent connections utilized by Outlook. Connection latency can be spot checked by clicking the reconnect button and observing the connection time. If a user has numerous calendars in their “My Calendars” folder, you will see those additional connections listed here.

• For Outlook for Windows clients, another common step to improve performances is to re-create the offline Outlook Data File (.ost). After completing this, start Outlook and see if the issue(s) exists.

• For all clients, but in particular Macintosh and other Exchange Web Service based clients, limiting the number of items in the Inbox root folder, the Deleted Items folder, and the Sent Items folder to under 5,000 will improve client performance.

• The native calendaring application for iOS/Android has known compatibility issues with Office 365 (Exchange) - it is highly recommended that you only use a Microsoft client to interact with your Office 365 account. When modifying calendar items via the native calendaring application, calendar item corruption can occur. It is recommended that the native iOS calendaring application only be used for calendar viewing. View supported clients | best practices | Microsoft issues.

• Microsoft recommends disabling scanning of Outlook files as outlined here.

Other Resources

• Microsoft: Best practices for using Office 365 on a slow network
• Microsoft: Outlook performance is slow in the Office 365 environment
• Microsoft: "Outlook not responding" error or Outlook freezes when you open a file or send mail
• Microsoft: With many items in a Cached mode .ost file or .pst file folders, Outlook performance issues may occur
• Microsoft: Troubleshoot performance issues in Outlook
• External: Clean up mailbox