Maybe you should try clearing the data and cache on the app, thatay speed it up a bit.
Functional mailbox
We do have a groupmailbox. Which my team of almost 40 people have an access to. At a time almost 12-15 people are in the shift and all of us have full access. We logon to our outlook and this mailbox is also added there. So we can check mails coming into this . However now for past weeks the mailbox is very slow. None of us can search anything or the folders are not updating etc. As per out IT the reason being there are too many people who has an access to the mailbox. For example there are 15 people in a shft and all of us access the group mailbox at the same time . But how can that be an issue ? isnt the purpose of group mailbox that number of people can access the mailbox together ? Also the size is only 60% used.