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 2013 and 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.
• 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.

Method 2: 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:
• In Outlook 2013 or Outlook 2010, in the Office Ribbon, click Send/Receive, click Send/Receive Groups in the Send & Receive group, and then click Download Address Book.
• In Outlook 2007, click Tools, click Send/Receive Settings, and then click Download Address Book.
2. In the Choose address book area, make sure that \Offline Global Address List 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 can't 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 fully qualified domain name (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 isn't 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 Microsoft Knowledge Base article:
   
   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 3: Check the OST file size
Make sure that offline OST file in Outlook isn't 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 Properties.
2. Click Folder Size, and then 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 7 or Windows Vista: %userprofile%\AppData\Local\Microsoft\Outlook
• Windows XP: %userprofile%\Local Settings\Application Data\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 4: 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 5: 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 2013 and 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.

Note You can also use the Office Configuration Analyzer Tool (OffCAT) to detect Outlook add-ins. For more info about OffCAT, see the following Microsoft Knowledge Base article: 2812744
(http://support.microsoft.com/kb/2812744/ )
Office Configuration Analyzer Tool (OffCAT) information

• In Outlook 2007:
1. On 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.

Method 6: 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 search box on the Start menu:
outlook.exe /safe
If performance improves, check again to make sure that you disabled any Outlook add-ins that may affect performance.

Method 7: 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, and then click the gear icon in the upper-right corner.
2. Click Tools, and then click Options.
3. Click Personal, and then clear the integration check boxes in the Personal information manager area.
4. Click OK, and then restart Outlook and Lync.

Method 8: 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 9: Check Group Policy settings
If your organization’s Exchange environment is governed by Group Policy, 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 10: 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 "Programs and Features" or Add/Remove Programs, and then select your version of Office from the list of programs.
2. Click Change.
3. Click Repair, and then click Continue.

Method 11: Cache additional mailboxes
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
(http://support.microsoft.com/kb/322756/ )
in case problems occur.

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, Outlook 2007 only caches non-mail folders. Mail folders use an online connection to the Exchange Online servers. To help improve performance, cache the mail folders of the additional mailboxes. 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 one of the following registry subkeys, depending on the version of Outlook that you're running:
• In Outlook 2013: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook
• In Outlook 2010: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook
• In Outlook 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\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 1, and then click OK.
9. Exit Registry Editor, and then start Outlook.
   
   Note By default Outlook 2010 caches all folders for additional mailboxes, regardless of folder type.

Method 12: Decrease the synchronization frequency for Outlook 2013 items or Outlook 2010 items

1. In Outlook 2013 or Outlook 2010, click the File tab, click Options, click Advanced, and then click Send/Receive.
2. Click the All Accounts option, and then click Edit.
3. In the navigation pane (left side), click SharePoint.
4. Under Send/Receive Options, clear the check boxes next to the SharePoint sites that you don't want to sync with Outlook, and then click OK.
5. Click New, name the new group, and then click OK.
6. Click the SharePoint button on the left side.
7. Select the SharePoint sites that you want to limit, and then click OK.
   
   Note You must clear all SharePoint syncs and RSS feeds that you don't want to limit. By default, new groups include all RSS feeds and all SharePoint sites.
8. Select the newly created group.
   
   The settings in the Send/Receive Group windows let you customize the synchronization rate of the new group.