Troubleshoot and solve Outlook performance issues

When users migrated to Office 365 complain about performance it’s mostly because they are using many shared mailboxes and calendars or have a big personal mailbox (5GB+). However, with some simple tricks you can increase performance quite a bit for them.

Tips to increase performance

  1. First of all, you can disable Hardware Graphics Acceleration in Outlook because most graphic cards cannot correctly handle this setting. Especially switching between calendars and mailboxes can increase performance by disabling this setting. This can be done in Outlook 2013/2016 by going to file, options and select the advanced tab.pic1
  2. If users are using many shared mailboxes or public folders, it is highly recommended to de-check the download shared folders option in Outlook. By default, Outlook wants to cache every folder of every (shared) mailbox to your local OST file. You can disable this setting in your account settings.pic2
  3. Outlook 2013/2016 wants to cache 12 months of email if you use it in caching mode. Recommended is changing this to let’s say 3 months for a “normal” size mailbox.
  4. I have seen companies using antivirus solutions scanning every file opened by a user. However, when opening Outlook, the OST file is being scanned as well causing very long startup times for Outlook.
  5. If you have Enterprise licenses for Office 365 use the Online Archive/In-Place-Archive functionality when users have large mailboxes. Online Archives will not cache emails to the local OST file so your Outlook profile will remain small when archiving correctly. As an administrator you can also help users by creating policies for auto archiving if desired.
  6. Outlook add-ins can significantly decrease performance and there are some of them known of doing so like the iCloud add-in to just give an example. Recommended is to disable any add-in which is not being used.
  7. Limiting the number of calendars in the “My Calendars” view will improve Outlook performance as well because Outlook will scan for changes every couple of minutes. Just de-check calendars if you are not using them constantly can help keep Outlook fast(er).
  8. Limit the number of emails in the root Inbox folder as this needs to be loaded directly when Outlook starts. Even creating a sub folder under the Inbox and move your emails to it will help increase performance.

Troubleshooting Outlook performance

  1. With the Office 365 Client Performance Analyzer which can be downloaded here
    This tool can be really helpful for troubleshooting your own network against Office 365. The scan can be performed on a PC with performance issues. With the above URL you can find what settings are measured and if a value is good or not.
  2. With the Microsoft Office Configuration Analyzer Tool (often referred to as OFFCAT) downloaded here
    This tool will scan a user’s Outlook profile completely and a lot of information can be retrieve afterwards when exported to a .offx file.
  3. Check the connection status for Outlook. This can be done by holding the Ctrl key and right-clicking the Outlook icon in the system tray near the clock. Select connection status. The number of outgoing connections to Office 365 will be visible here. There is not a really easy answer on what your average response time needs to be etc. but I think if you are generally under about 200-300 you should be doing ok.


Get everyone’s mailbox size with this handy Powershell cmdlet

  1. Connect to Exchange Online. If you don’t know how to do this, I included this in another post which can be found here
  2. Make sure you have a temp folder on your C drive and perform this cmdlet:
    Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | Select DisplayName,StorageLimitStatus,@{name=”TotalItemSize (MB)”;expression={[math]::Round(($_.TotalItemSize.ToString().Split(“(“)[1].Split(” “)[0].Replace(“,”,””)/1MB),2)}},@{name=”TotalDeletedItemSize (MB)”;expression={[math]::Round(($_.TotalDeletedItemSize.ToString().Split(“(“)[1].Split(” “)[0].Replace(“,”,””)/1MB),2)}},ItemCount,DeletedItemCount | Sort “TotalItemSize (MB)” -Descending | Export-CSV “C:\temp\AllMailboxes.csv” -NoTypeInformation
  3. Now check the AllMailboxes.csv file in your temp folder. It will contain several fields, but most important the mailboxes are sorted on size now with the biggest one on top.pic3

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