Managing Large Shared Mailboxes in Exchange Online – Performance Strategies and Trade-offs

Trying to think on how to tackle the same issue - New Outlook doesn't seem to have the same concept as an 'OST' file for it to sync to locally if you wanted to to try to mask performance that way....there is 'Offline' capabilities, but these do not appear to be like for like.

I've thought about what VasilMichev mentions - Archiving as a solution.  We have a shared mailbox that already has a retention policy on it of 6 months, but the data turn over in the mailbox has it hovering between 40-50 gb, with users (not yet on New Outlook) complaining about performance hangs.  I have thought about trying to enabling archiving for the mailbox to move things older to a month, and then a retention policy of 6 months on the archive....if that's even possible.