Disabling sync of shared mailboxes
If outlook is running slow after a shared mailbox is added then the cache settings in outlook need to be adjusted to be less ram and disk intensive for your computer.
In outlook go to File / Account Settings / Account settings
Then click your email address and click Change. In the next screen click More Settings
Lastly go to the Advanced Tab Uncheck Download Shared folders.
Press ok 2 times and restart Outlook. It will take a few moments as it purges the emails from your computer, but then Outlook should be more stable.