Outlook performance issues in a Cached Exchange Mode .ost or .pst file
Original KB number: 2768656
Symptoms
In Microsoft Outlook, you experience symptoms such as the following:
- When you use Cached Exchange Mode or an Outlook data (.pst) file, you notice decreased performance when you take certain actions.
- If the Outlook Inbox, Calendar, Tasks, Sent Items, or Deleted Items folders contain lots of items, you experience decreased performance.
- Calendar performance is inconsistent. For example, meeting updates might not be reflected in the primary, shared, or delegated calendar.
- Folders take a long time to appear or don't appear correctly.
- If your Outlook profile contains shared mailboxes and has caching enabled (Download Shared Folders is selected), you encounter folder synchronization issues, decreased performance, or other issues if the number of shared folders per mailbox exceeds 500. For more information, see Performance and synchronization problems when you work with folders in a secondary mailbox in Outlook. Additionally, errors are logged in the Sync Issues folder, and "9646" events are logged in the Application log.
- In extreme cases in which there are more than 10,000 folders, Outlook is very slow to open. This behavior occurs because of the time required to enumerate the large number of folders.
Cause
These issues might occur if a mailbox contains lots of folders or there are lots of items in any one folder. There's no hard limit to how many folders a mailbox can contain or how many items a folder can contain. However, you might experience decreased performance as the number of items approaches 10,000 calendar items, 10,000 folders, or 100,000 mail items per folder. Large numbers of recurring meetings or long-lived recurring meetings can also have an exaggerated effect on performance.
Resolution
To fix performance issues that affect your Calendar, Mail, or shared folders, use the following resolutions, as appropriate.
Resolutions for Calendar issues
To fix performance issues that affect your Calendar, try the following methods.
Method 1: Manage the growth of exceptions for recurring meetings
Not all calendar items have an equal effect on performance. One-time meetings have a relatively small effect, and long-lived, recurring meetings have a greater effect. A change that's made in an instance of a recurring meeting creates exceptions that are stored in the recurring meeting series. Every change to the meeting attributes, such as the subject, body, location, or time, creates a corresponding exception. Eventually, these exceptions inflate the size of the recurring meeting and impose a greater performance cost. You can mitigate this higher cost by using Appointment Recurrence settings to set an end time or maximum recurrence count. Then, you can create a new recurring meeting instead of extending the existing recurrence.
Method 2: Enable shared calendar improvements
Use the Turn on shared calendar improvements option for users who have edit permissions to a shared calendar to improve performance. When you enable this option, calendar actions are sent directly to the server instead of having to be synchronized from local storage. This makes conflict resolution more efficient.
For more information about how to activate and manage the shared calendar improvements, see How to enable and disable the Outlook calendar sharing updates
Method 3: Limit the Sync window
You can limit the synchronization window that’s used for calendar folders to reduce the number of items that are stored locally in your calendar folders. Doing this can improve performance.
Important
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
In Sync Window Settings, adjust the number of months of data that are synced for the primary shared calendars. To do this, add the following registry keys.
Note: This method applies to Version 1810 or a later version of Microsoft 365 Apps for enterprise (Click-to-Run).
| Description | Setting to enable Calendar Sync window |
|---|---|
| Registry path | HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode |
| Name | CalendarSyncWindowSetting |
| Type | REG_DWORD |
| Value | Value = 0 Inactive Value = 1 Primary Calendar folder Value = 2 All Calendar folders Defaults to 0 if not set. |
| Description | Setting to control the number of months in the Calendar Sync window |
|---|---|
| Registry path | HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode |
| Name | CalendarSyncWindowSettingMonths |
| Type | REG_DWORD |
| Value | Value = Select a decimal value to set the number of months in the Calendar sync window. For example, select 1, 3, 6, or 12. Defaults to 6 if not set. |
| Description | Setting to control whether to keep all recurring items instead of filtering them |
|---|---|
| Registry path | HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode |
| Name | CalendarSyncWindowAllRecurring |
| Type | REG_DWORD |
| Value | Value = 0: Only recurring meeting series that have an end date that is either in the future or falls within the current calendar sync window setting will be synced. Value = 1: All recurring meeting series will be synced, regardless of the end date. Defaults to 0 if not set. |
| Explanation | By default (if the value isn't set or is set to 0), recurring meeting series that have an end date that is either in the future or falls within the current calendar sync window setting will be synced. For example, if today's date is May 3, 2022, and the calendar sync window is set to 1 month, all recurring meeting series that have an end date on or after April 3, 2022, will be synced. If the end date is outside the sync window setting, the recurring meeting series will be removed from the .ost file. To sync all recurring meeting series regardless of the end date, set the CalendarSyncWindowAllRecurring value to 1. |
These registry keys update the sync restriction so that the client that's using Cached Exchange Mode downloads fewer Calendar items, even for a Calendar that has multiple years of history on the server. These keys don't clean up older Calendar content that was already downloaded. However, this method can be effective if you're willing to clear offline items and resync your Calendar (instead of bulk deleting old items). If you already have the Calendar in the profile, you have to clear the Offline Items after you set the registry keys and restart Outlook.
To clear offline Calendar items, follow these steps:
- Open the Calendar pane in Outlook, and right-click the Calendar folder.
- Select Properties.
- On the General tab, select Clear Offline Items.
- Select OK.
Resolutions for Mail issues
If you have folders whose content is approaching the limit of 10,000 calendar items, 10,000 folders, or 100,000 mail items, and these items are stored 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 that archive or create retention policies to automatically dispose of older items.
Archive Mail items
If you have enabled Online Archive Mailbox, archive items to the Online Archive Mailbox.
Apply a retention policy
To delete older items from the Mail or Calendar folder, apply a retention policy. For example, any item that is not modified within one year moves to the Deleted Items folder.
Resolution for Shared Folders issues
To fix performance issues that affect shared folders, try the following method.
Limit the Sync window
To reduce the number of items that are stored locally, limit the synchronization window that's used for shared folders. Doing this can improve performance.
Important
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
To adjust the number of days of data that are synced for shared folders, add the following registry values.
| Description | Setting to enable Shared Folders window |
|---|---|
| Registry path | HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode |
| Name | SharedFolderSyncWindowSettingDays |
| Type | REG_DWORD |
| Value | Value = Select a Decimal value to set the number of days (1-365) for the shared folders sync window. |
Run Microsoft Support and Recovery Assistant
Note
Microsoft Support and Recovery Assistant scenarios aren't available for new Outlook for Windows.
You can use Microsoft Support and Recovery Assistant to diagnose both Calendar and Mail issues that affect Outlook. To download and install the Assistant, see About Microsoft Support and Recovery Assistant.
To determine the cause of the performance issues, start the Assistant, and run the Outlook Diagnostic in the Advanced Diagnostics section.
More information
To view the item count in an Outlook Calendar when you use Cached Exchange Mode, use the Calendar Properties dialog box:
- In Outlook, open the Calendar pane, and then right-click the Calendar folder.
- Select Properties.
- On the General tab, select Show total number of items.
- Select the Synchronization tab.
- View the count under View statistics for this folder.
Outlook uses an .ost file only if the Exchange email account is configured to use Cached Exchange Mode. If 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 don't have high-item-count folders in a .pst file, any performance issues that you experience might be occurring on the server. For more information, see You may experience application pauses if you have a large Outlook data file.