Hi,
Does this issue happen to the Exchange account folders or IMAP account folders?
Do you have Cached Exchange Mode enabled? If not, please try enabling cached exchange mode and then check if this issue persists. Additionally, please makes sure "Download shared folders" is not enabled.
In addition, please check the file size of your Outlook data file. If they are too large and exceeding the size limit, your Outlook data files may become corrupted and may cause Outlook performance issue.
If above doesn't help, please also try start Outlook in safe mode to see if this issue persists. To do this, press Windows + R to open the Run command, type outlook /safe and press Enter.
For more troubleshooting steps regarding Outlook slow performance, please refer to the following link:
https://connect.rhodes.edu/helpdesk/kb/article/968
Please Note: Since
the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Regards,
Steve Fan
TechNet Community Support