How can I backup Outlook for Mac 2016 on my mac?
Let the export complete. The internal storage of mails in Outlook 2016 for Mac is a cached version for all imap / google accounts that changes internally from version to version. 

The export yields a far more standard and stable data dump which serves the purpose of being importable at a later date. 

If you have local folders - on my Mac then things are slightly more complicated. Perhaps that's part of the slower export?