Hi OliverMunday,
This may because the shared boxes’ size is too large. By default, the shared box items are downloaded into your local cache OST file if your Office 365 mailbox is configured in Exchange cached mode. A too big OST often leads to bad Outlook performance.
To determine this, please try to export the shared items to a PST file as backup and then delete some old items to reduce the size. After that, you can also let the users view shared items via opening the PST file in the Outlook client. Feel free to let
us know if you encounter any problem during the progress.
If the issue persists after that, please let us know and we will further investigate it.
Regards,
Larry