This post I made in the discussion from phpBB 3 Importer (old) might be of interest to you:
Yes, this was an older version of in the import script but might still be relevant.
It turns out the import was more like just over 750K posts and 150K topics including private messages.
The sidekiq’s execute background tasks created by each imported post and it’s always worth a little look in /sidekiq to see if you have a backlog. That’s the reason I mention “at least 25 sidekiq’s running” to keep that those tasks down.
My import ran for well over 36 hours on a 64GB Memory, 20 Core Processor machine which I ended up using.