Fix Outlook Performance Issues with too many Items or Folders in Cached Mode

At times, you experience that your Outlook’s performance has degraded. When you take certain actions, they’re taking longer than usual. For example, delay in emails transmission, slow or incomplete search results, delayed calendar updates, etc. This usually happens when you have an oversized mailbox, complex folder structure, or multiple shared calendars. This guide provides some effective solutions to reduce the mailbox or OST file size, thus improving Outlook performance.

Threshold Limit of Different Items/Folders in Microsoft Outlook  

Before selecting a fix, refer to the table below to find out which threshold limit you are exceeding.

Symptoms of Performance Issues and their Fixes

Refer the table below to find the right fix based on what you are experiencing:

Methods to Resolve Outlook Performance Issues due to Large Mailbox in Cached Mode

Below, we have discussed different methods which you can use to reduce mailbox or Outlook data file size and resolve performance problems in Outlook.  

Method 1: Archive Mail Items

You can use the archive option in Outlook to move mail items - based on a specific date – to a local PST file. This helps reduce the number of mail items and folders from the mailbox and OST file.

You may either manually archive the items or enable the auto-archive feature. The steps to manually archive items are as follows:

Outlook 2010, 2013, 2016, 2019 and Microsoft 365 (Classic Outlook):

• File > Info > Cleanup Tools > Archive.

In older Outlook versions:

• File > Info > Tools > Clean Up Old Items.

• Click Archive this folder and all subfolders. 
• Select the folder you want to archive.
• Select a date under Archive items older than.

• Check the Include items with "Do not AutoArchive" checkbox to archive all items.
• Click OK.

Method 2: Apply Retention Policy

You can also apply or assign retention policies to mailbox for automatically archiving or deleting emails after a particular period. This helps in keeping the mailbox size in check.

You can apply a retention policy on email folders by following these steps:

• Launch the web browser and sign in to your email account using Outlook Web App URL.
• Enter your username and password.
• Click Sign in.
• In the folder pane, right-click the message or folder on which you want to apply a policy.
• Select Assign Policy.

• Choose the Archive Policy or retention policy, under the Labels, you want to assign to the mail folder and all mail items in it.

Method 3: Adjust Sync Settings

When your email profile is set up in Cached Exchange Mode, Outlook provides a cached mode sync slider setting to adjust the number of mail items or folders to store in the OST file. You can adjust this slider to avoid storing or exceeding the number of mail folders or items. To do so:

• Launch Outlook. Go to File > Info > Account Settings > Account Settings.

• Choose your email profile and click Change.
• Under Offline Settings, adjust the Mail to keep the offline slider. If it's currently set to All, move it to 3 years or lower to reduce the number of mail items, calendar items, and mail folders in your offline OST file.

• Once adjusted, click Next and then click OK.
• Re-launch Outlook.

As an alternative, you can adjust the number of months or years of data to be synchronized for shared calendars by editing the registry keys. Here’s how:

• Press Windows+R to open the Run utility.
• Type Regedit and press Enter.
• Go to the this location:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode

• Set or modify the CalendarSyncWindowSetting value to 1 (Primary Calendar Folder) or 2 (All Calendar folder) based on your requirements.
• Next, create or modify the CalendarSyncWindowSettingMonths registry with values 1, 3, 6, or 12 (in months). This will help reduce the number of items in the calendar.

Method 4: Clear Offline Items

Clearing the Offline Items may also help reduce the local OST file size, thus improving the Outlook performance. To do so:

• Find and confirm the folders having too many mail items or folders. For this,

  • Right-click on the mail folder and choose Properties.
  • Click the Synchronization tab and check the number of items the folder contains.

• Once you have found and confirmed the folders having too many mail items, click the General tab and then click Clear Offline Items.

• This will remove the offline mail items. Click OK and restart Outlook.

You may also open the Calendar properties to Clear Offline Items from the Calendar folder.

Method 5: Disable the Download Shared Folders Option

Outlook stores shared folders data in the OST file when the Download Shared Folders option is enabled. This will considerably increase your OST file size. As a result, it slows down Outlook and lead to other performance issues. You can disable this option to prevent downloading of shared folders data. Here’s how:

• Open Outlook. Go to File > Account Settings > Account Settings.

• In the window that opens, choose Data Files and select the Outlook data file (OST).

• Choose Settings and then go to the Advanced tab.

• Here, uncheck the ‘Download shared folders’ option, click Apply and then click OK.
• Restart Outlook.

Method 6: Compact OST File

You can also compact the OST file. It removes the unused space left behind by previously deleted items, thus helps reduce the OST file size. For compacting the OST file:

• Close Outlook and open Control Panel.

• Search and click on the Mail.
• Click Data Files…
• Select the OST associated with your profile and click Settings.
• Click Advanced and then click Outlook Data File Settings…

• Click Compact Now. This compresses and reduces the OST file size.

Method 7: Rebuild Search Index

When mailbox becomes large, the search index may not work properly, affecting the search results. In this case, you can rebuild the index. Here’s what to do:

• Open Outlook.

• Go to File > Options.
• Select Search from the left pane.

• Click Indexing Options.
• Click Advanced.

• Click Rebuild.

• Click OK to confirm and start the rebuild process.
• Keep Outlook open while the index is being rebuilt.
• Depending on the mailbox size, the process can take anywhere from 30 minutes to several hours.

What to do when manual methods fail to fix this issue?

The methods we have mentioned above can help reduce OST file size and improve your Outlook performance. At times, situations arise where your email client becomes slow, unstable, or even inaccessible. Due to this, you’re not able to perform the troubleshooting methods. In such a situation, you can extract data from your OST file and save it into multiple smaller PST files. For this, Stellar Converter for OST is a perfect tool. It works directly on the OST file without opening Outlook.

You can use this tool in the below scenarios:

• Outlook is too slow or crashes before you can apply fixes.
• The OST file has reached its defined (50 GB) size limit.
• Outlook is unable to compact the OST file.
• OST has become inaccessible or orphaned.

Conclusion

You may experience performance degradation of other issues in Outlook if there are too many items or folders in the mailbox. You can decrease the OST file size to improve its performance. For this, you can follow the methods mentioned in this guide.

In case your OST file is not inaccessible, you can use Stellar Converter for OST. This tool can extract mail items from large, inaccessible OST file and save them in multiple, smaller PST files. You can then import these files into your email client.