Troubleshooting Outlook Performance Issues

Summary

This guide helps you troubleshoot and resolve: Troubleshooting Outlook Performance Issues. Follow the steps below to fix the issue.

Common Causes

Check Microsoft 365 Service Health

Before troubleshooting your local Outlook installation, confirm that Microsoft 365 services are operating normally. Server-side issues can cause Outlook to run slowly or appear unresponsive.

• Sign in to the Microsoft 365 Admin Center.
• Navigate to Health > Service health.
• Check for any active incidents or advisories affecting Exchange Online.
• If a service degradation is reported, wait for Microsoft to resolve the issue before proceeding with local troubleshooting.

Step-by-Step Troubleshooting

1. Check for Updates

   Ensure Outlook and Microsoft 365 Apps are fully updated. Performance fixes are often included in monthly updates.

   • Open Outlook.
   • Go to File > Office Account > Update Options > Update Now.
   • Restart Outlook after the update completes.

2. Run Outlook in Safe Mode

   Safe Mode starts Outlook without add-ins and extensions. This helps determine if an add-in is causing the slowdown.

   • Close Outlook completely.
   • Press Win + R, type outlook /safe, and press Enter.
   • If Outlook performs normally in Safe Mode, a third-party add-in is likely the cause.

3. Disable Add-ins One by One

   If Safe Mode resolves the issue, identify the problematic add-in by disabling them individually.

   • Go to File > Options > Add-ins.
   • At the bottom, next to “Manage:”, select COM Add-ins and click Go….
   • Uncheck all non-essential add-ins.
   • Click OK and restart Outlook.
   • If Outlook is fast, re-enable add-ins one at a time, restarting Outlook after each, until the slowdown returns.

   Add-ins labelled as “Required” should not be disabled.

4. Check Mailbox Size and Archive Old Items

   Large mailboxes can degrade Outlook performance. Review your mailbox size and archive older emails to reduce the load.

   • Go to File > Info.
   • Review the Mailbox Cleanup section or use File > Tools > Mailbox Cleanup to view mailbox size.
   • Archive items older than a specific date using File > Info > Tools > Clean Up Old Items.
   • Move large emails with attachments to an archive folder or delete them if no longer needed.

   Exchange Online mailbox size limits vary by subscription plan. Check your plan details in the Microsoft 365 Admin Center if you are approaching your storage quota.

5. Compact PST Files

   Over time, Personal Storage Table (.pst) files can become fragmented and oversized, slowing Outlook performance.

   Important: Back up your PST file before compacting or repairing it. See Backing Up and Restoring Outlook Data Files (.pst/.ost).

   • Go to File > Account Settings > Account Settings > Data Files.
   • Select the PST file and click Settings.
   • Click Compact Now.
   • Ensure you have at least 10% of the file size in free space on the drive before compacting.

6. Reduce OST Offline Mail Settings

   Offline Storage Table (.ost) files store a local copy of your Exchange mailbox. Keeping too much mail offline can cause performance issues.

   Important: Back up your OST file before changing offline mail settings or cache configuration. See Backing Up and Restoring Outlook Data Files (.pst/.ost).

   • Go to File > Account Settings > Account Settings.
   • Double-click your email account.
   • Under “Mail to keep offline”, move the slider to a shorter timeframe (for example, 3 or 6 months) instead of “All”.
   • Click Next > Finish and restart Outlook to recreate a smaller OST file.

7. Configure Cached Exchange Mode

   Cached Exchange Mode keeps a local copy of your mailbox for faster access. Disabling it temporarily can help determine if the local cache is causing performance issues.

   • Go to File > Account Settings > Account Settings.
   • Double-click your email account.
   • Clear the checkbox for Use Cached Exchange Mode.
   • Click Next > Finish and restart Outlook.
   • If performance improves, re-enable Cached Exchange Mode and reduce the “Mail to keep offline” slider.

8. Repair Office Installation

   Corrupted Office program files can cause Outlook to run slowly or crash.

   • Close all Office applications.
   • Open Settings > Apps > Installed Apps.
   • Find Microsoft Office, click the three-dot menu, and select Modify.
   • Choose Quick Repair. If the issue persists, run Online Repair.
   • Restart your computer after the repair completes.

9. Repair Data Files with ScanPST

   Use the Inbox Repair Tool to fix corrupted PST or OST files.

   Important: Back up your data file before running ScanPST. See Backing Up and Restoring Outlook Data Files (.pst/.ost).

   • Close Outlook.
   • Locate SCANPST.EXE (typically in C:\Program Files\Microsoft Office\root\Office16).
   • Run the tool, select your data file, and click Start to begin the repair.

10. Rebuild Windows Search Index

    A corrupted Windows search index can make Outlook search slow and unresponsive.

    • Open Control Panel > Indexing Options.
    • Click Modify and ensure Microsoft Outlook is selected.
    • Click OK, then click Advanced.
    • Under the Index Settings tab, click Rebuild. This process may take some time depending on the amount of data.

11. Disable Hardware Graphics Acceleration

    Graphics rendering conflicts can affect Outlook performance, particularly on systems with older or incompatible display drivers.

    • Go to File > Options > Advanced.
    • Scroll to the Display section.
    • Check Disable hardware graphics acceleration.
    • Click OK and restart Outlook.

12. Create a New Outlook Profile

    A corrupted Outlook profile can cause crashes and persistent slowness.

    • Close Outlook.
    • Open Control Panel and search for Mail (Microsoft Outlook).
    • Click Show Profiles.
    • Click Add…, name the profile (for example, “Outlook Test”), and set up your email account again.
    • Select “Prompt for a profile to be used”, restart Outlook, and choose the new profile.
    • If performance improves, set the new profile as the default.

13. Check Antivirus Exclusions

    Antivirus software scanning Outlook data files in real time can cause significant slowdowns.

    • Open your antivirus management console.
    • Add exclusions for outlook.exe and your PST/OST file locations.
    • Restart Outlook and test performance.

Large Attachments and Send/Receive Settings

Sending or receiving very large attachments can cause Outlook to hang or time out.

• Avoid attaching files larger than 20 MB directly to emails. Use OneDrive or SharePoint links instead.
• Review Send/Receive groups under File > Account Settings > Account Settings > Email > Change > More Settings > Advanced to ensure download limits are appropriate for your connection.

When to Seek Further Assistance

If you have completed the steps above and Outlook performance does not improve, the issue may be related to hardware, operating system corruption, or a complex Exchange connectivity problem. Open a Ticket for further support.

Still Having Issues?