Outlook performance is slow in the Office 365 environment

When you try to use Microsoft Outlook together with Microsoft Office 365, the following issues may occur:  

• Outlook responds slowly when you try to open email messages.
• When you send an email message, the message sits in the Outbox folder for a long time.
• Outlook responds slowly when you try to insert an attachment. Frequently, you receive the following warning message:
  Outlook is retrieving information from the server

This issue may occur for many reasons. This issue may be caused by a Microsoft Online server issue. Sign in to the Microsoft Office 365 portal with your administrator credentials, and then click Service Health on the Admin Home page to determine whether other customers are experiencing the same issue. If other customers are experiencing the issue, the issue should go away when the service-interrupting event is resolved.

Also, determine whether there are any local network issues that might be causing the problem, such as issues with your local proxy server, firewall, or Internet service provider.

To determine the specific cause and to resolve the issue, use the methods that are described in this article.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
To troubleshoot this issue, use one or more of the following methods:

Method 1: Enable Cached Exchange Mode

You can try to improve performance by making sure that Cached Exchange Mode is enabled. To do this, follow these steps, depending on the version of Outlook that you are using:

• In Outlook 2007:
  1. Click the Tools menu, and then click Account Settings.
  2. In the E-mail Settings window, select the user’s Exchange Server account, click Change, and then make sure that the Use Cached Exchange Mode check box is selected.
• In Outlook 2010:
  1. Click the File tab, click Account Settings, and then click Account Settings.
  2. On the E-mail tab, select the user’s Exchange Server account, click Change, and then make sure that the Use Cached Exchange Mode check box is selected.

Method 2: Determine global catalog usage

By default, if Outlook cannot access the Offline Address Book, it uses the online global address list (GAL). This can slow performance. To determine whether this is the case, click the Address Book icon in Outlook, right-click the GAL, and then click Properties. If the properties show the fully qualified domain name (FQDN) of a server instead of a local folder path, Outlook is using the online GSL instead of the Offline Address Book. If this is the case, you can download the Offline Address Book.

Method 3: Download the Offline Address Book

Make sure that you can download the Offline Address Book (OAB). To do this, follow these steps:

1. Take one of the following actions:
   1. In Outlook 2007, click Tools, click Send/Receive Settings, and then click Download Address Book.
   2. In Outlook 2010, in the Office Ribbon, click Send/Receive, click Send/Receive Groups in the Send & Receive group, and then click Download Address Book.
2. In the Choose address book area, make sure that \onsite AL is listed as an option.
3. Determine whether Outlook is directly using a Microsoft Online global catalog (GC) server for online global address list (GAL) lookups. By default, if Outlook cannot access the OAB, it uses the online GAL. To determine whether this is the issue, right-click the GAL, and then click Properties. If the FQDN of a server is listed, Outlook is using the online GAL. If the path of the OAB file on the computer is listed, Outlook is using the OAB, and the Outlook slowness is not caused by OAB issues. If Outlook is using the OAB, you can help troubleshoot the issue by following the steps that are described in the following article in the Microsoft Knowledge Base :
   
   2429946  (http://support.microsoft.com/kb/2429946/ )  How to troubleshoot the Outlook Offline Address Book in an Office 365 environment
   
   Use the procedure in this article as a baseline to make sure that the client downloads the correct OAB.

Method 4: Check the OST file size

Make sure that Outlook’s offline OST file is not too large. A large OST file may indicate corruption or fragmentation of the file. This can affect performance.

1. In Outlook, right-click the mailbox in the navigation pane, and then click Data File Properties.
2. Click Folder Size, and note the Total size value on the Local Data tab.
3. Open the Outlook system folder by opening the following folder path, depending on the operating system:
   
   Windows XP: %userprofile%\Local Settings\Application Data\Microsoft\Outlook
   
   Windows Vista or Windows 7: %userprofile%\AppData\Local\Microsoft\Outlook
4. Note the size of the OST file for the user’s Outlook profile. If the OST file is more than 3 to 4 gigabytes (GB) larger than the Total size value, the file may be corrupted or fragmented. Close Outlook, rename the OST file, and then reopen Outlook to re-create the OST file.

Method 5: Reduce the number of items in critical folders

If you continue to have problems, make sure that there are not too many items saved in the Calendar, Contacts, Inbox, and Sent Items folders. If the Inbox folder or the Sent Items folder contains more than 20,000 items, or if the Calendar folder or the Contacts folder contains more than 5,000 items, the performance of Outlook can be affected.

Method 6: Disable add-ins

If Outlook is already using the Offline Address Book, or if the problem continues after you download the Office Address Book, an add-in may be causing Outlook to respond slowly. In this case, try to disable add-ins in Outlook. To do this, follow these steps, depending on the version of Outlook that you are using.

• In Outlook 2007:
  1. Click the Tools menu, click Trust Center, and then click the Add-ins tab.
  2. In the Add-ins window, disable or remove any add-ins that might be causing the performance problems, such as those that scan each message that you receive or those that scan index messages.
• In Outlook 2010:
  1. Click the File tab, click Options, and then click the Add-ins tab.
  2. In the Add-ins window, disable or remove any add-ins that might be causing the performance problems, such as those that scan each message that you receive or those that scan index messages.
     

Method 7: Start in safe mode

If the problem continues, try restarting Outlook in safe mode by running the following command from the Run box or from the Start Search box on the Start menu:

If performance improves, check again to make sure that you disabled any Outlook add-ins that may affect performance.

Method 8: Disable Lync Online integration

If Outlook is integrated with Microsoft Lync Online in your Office 365 environment, the integration may be affecting Outlook’s performance. To disable Lync Online integration with Outlook, follow these steps:

1. Open Lync Online, and then click the gear icon in the upper-right corner.
2. Click Tools, and then click Options.
3. On the Personal tab, clear the integration check boxes in the Personal information manager area.
4. After you save the changes, restart Outlook and Lync Online.

Method 9: Disable email scanners

Email scanners, search programs such as Google desktop, and other services that scan the Outlook mailbox may also affect the performance of Outlook. Try disabling any such programs or services, and then see whether Outlook performance improves. You can determine which programs and services are currently running by pressing CTRL+ALT+DEL to open Task Manager. Some antivirus software can also scan incoming email messages. Make sure that you talk to the IT administrator before you disable any antivirus software on your computer

Method 10: Verify Group Policy settings

If your organization’s Exchange environment is governed by Group Policy, you should determine whether there are any group policies specific to Outlook that apply to the users who are experiencing the poor performance. If there are, determine whether you can reproduce the performance issues from another computer that does not receive that Group Policy setting. By doing this, you can determine whether the policy is contributing to the poor performance. If this is the case, you can amend or eliminate the policy setting.

Method 11: Repair Outlook

If the problems continue, try to repair Outlook on the computer where the problems occur. To do this, follow these steps:

1. In Control Panel, open the Programs and Features or Add/Remove Programs window, and then select your version of Office from the list of programs.
2. Click Change.
3. Click Repair, and then click Continue.

Method 12: Cache additional mailboxes

1. If an additional mailbox is added to the user's Office 365 mailbox (for example, a shared or resource mailbox), Outlook may slow down when it tries to connect to the other mailbox. By default, this is an online connection to the Exchange Online servers. To help improve performance, cache the additional mailbox. To do this, follow these steps:
   1. Start Registry Editor. To do this, click Start, click Run, type regedit, and then click OK.
   2. Expand the following registry subkey:
      • In Outlook 2007: 
        HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook
      • In Outlook 2010: 
        HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook
   3. Right-click Outlook, point to New, and then click Key.
   4. Type Cached Mode, and then press Enter to name the new key.
   5. Right-click Cached Mode, point to New, and then click DWORD Value.
   6. Type CacheOthersMail, and then press Enter to name the new value.
   7. Right-click CacheOthersMail, and then click Modify.
   8. In the Value data box, type 00000001, and then click OK.
   9. Exit Registry Editor, and then start Outlook.
   10. Take one of the following actions:
       • If you are running Outlook 2007, on the Tools menu, click Account Settings.
       • If you are running Outlook 2010, in the Office Ribbon, click the File tab, click Account Settings, and then click Account Settings.
   11. Click the Exchange Server account, and then click Change.
   12. Click More Settings.
   13. On the Advanced tab, click to clear the Download shared folders (excludes mail folders) check box.
   14. Click OK, click Next, and then click Finish.