I first reported this back in October 2019 and that thread can be found back in the Pre-1.9 Bug Report section.
Both the afpub and the text file in question were uploaded back then. (Hang_text1.afpib and Bible.txt)
It's been 2 years since my last post on this subject, so I decided to give it another test with the latest beta (1.10.2.1187)
Now it takes 55 minutes before I get control back, so there's been some improvement over the previous 1,5 hours.
That said, having to wait almost a full hour to auto flow pure text is not satisfactory. Admittedly, it's 1800+ pages, but other users reported the process taking a handful of minutes, back in 2019.
I assume this process is mostly CPU and memory intensive. I should be well covered there (6-core i7 and 64GB RAM), so I'm left wondering what's causing this.
Any thoughts, devs?
Edit: This should probably have been posted in the beta section. Feel free to move.