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—particularly in switching into and out of a folder that has lots of items.
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 like the following in Outlook:
Folders are not displayed correctly, or they take a long time to appear, especially in cached mode.
If your Outlook profile has shared mailboxes with caching enabled (Download Shared Folders is selected), performance issues occur if the number of folders across those mailboxes exceeds 500, as described in KB article 3115602. Additionally, errors are logged in the Sync Issues folder and 9646 events are logged in the Application log.
In extreme cases, when 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 2016, Outlook 2013 and Outlook 2010:
100,000 items per folder 500 folders total
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 Office Configuration Analyzer Tool (OffCAT) to determine which folder or folders are near or above this item limit. To download and install the OffCAT tool, go to the following Microsoft website:
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.