Same problem here. Freeze on file save. Small file size. While painting over a line drawing imported from Affinity Designer. Freezes on save status bar. 2017 iMac, OS 10.13.6, 24gigs RAM, internal Mac SSD. Only way to resume is to force quit the application. Happens repeatedly. Work lost. Annoying and nerve-wracking.
I just want to add to this discussion that it probably would have been better if you had master-page functionality before releasing the beta. It's really fundamental to a publishing application to be able to add new pages to a document that are based on a master, and then edit those pages without it retroactively applying those edits to the master and all previous pages based on the same master! I also want to say that I understand the extraordinary amount of work that goes into this, and I'm pulling for your success. We're all looking forward to the day when we no longer have Adobe in our lives. I think that day will come sooner if you nail down the basics first, and master pages is one of those basics. Thanks.