Outlook performance issues when there are too many items or folders in a Cached mode .ost or .pst file folder

Applies to: Outlook 2016Outlook 2013Microsoft Outlook 2010 More


If you have lots of items in any single folder, you may experience symptoms like the following in Microsoft Outlook:
  • When you use Cached Exchange Mode or an Outlook data (.pst) file, you notice performance issues when you perform certain actions.
  • You experience decreased performance in Outlook if the Inbox, Calendar, Tasks, Sent Items, and Deleted Items folders contain lots of items.

If you have lots of mail folders, you may experience performance issues such as the following:
  • Folders are not displayed correctly, or they take a long time to appear, especially in cached mode.
  • If your Outlook profile has shared mailboxes and has caching enabled (Download Shared Folders is selected), folder synchronization issues, performance issues, and other problems occur if the number of shared folders per mailbox exceeds 500, as described in KB 3115602. Additionally, errors are logged in the Sync Issues folder and "9646" events are logged in the Application log.
  • In extreme cases, if there are more than 10,000 folders, Outlook is very slow to open. This behavior occurs because it takes a long time to enumerate the folders.


This problem may occur if you have folders that exceed the limit for the number of items per folder or if you have too many folders total. The item and folder limits depend on the version of Outlook, as follows.

Outlook 2019, Outlook 2016, Outlook 2013 and Outlook 2010:
100,000 items per folder
500 folders total

Outlook 2007:
50,000 items per folder
500 folders total


If you have exceeded the maximum of 500 folders total, reduce the number of mail folders that you have.

If you have folders that exceed the limit for the number of items in an Outlook data (.pst) file or an offline Outlook data (.ost) file, move items from the larger folders to separate or smaller folders in the same mailbox or data file. Optionally, if you have a Microsoft Exchange online archive, you can move items to it.

You can use the Support and Recovery Assistant for Office 365 to diagnose issues that affect Outlook (not only Office 365).

To download and install the tool, go to the following Microsoft website:

Download Microsoft Support and Recovery Assistant for Office 365

In the tool, run the Outlook Diagnostic under the "Advanced Diagnostics" section to determine cause of the performance issue.

More Information

Outlook uses an .ost file only if the Exchange email account is configured to use Cached Exchange Mode. When Outlook is configured to connect to the Exchange mailbox in online mode, an .ost file is not used. If your Outlook client is connected to Exchange in online mode, and you do not have high item-count folders in a .pst file, the performance issue may be on the server.

The following articles list Exchange server performance issues that may occur when a mailbox has high item counts and when restricted views are being used. Additionally, the articles explain how the overall client user experience can be affected by these performance issues.