We've users in a number of departments complaining about slow performance whilst one or two shared mailboxes. These mailboxes aren't particularly large in size (compared to the thousand other shared mailboxes we have) and we've had our ISP confirm there are no issues with the network.
When the users start Outlook in safe mode, the performance is improved immensely. The majority of their add-ins are still available and the only difference I can see is in the Connection Status. When using Outlook normally, most of the connections have a Sess Type listed as Foreground, whereas in safe mode, they're listed as Background.
I've not been successful in finding out what Foreground and Background are and how to configure Outlook to use one of these Sess Types.