So - I was just playing for about 15 minutes, this time in windowed mode.
I'm currently in Mecidi and went into Ariel Alchemicalery, walked a couple of steps and the it froze again - just to be sure I waited around 10 minutes, no change.
No fight, no NPC visible, no combat mode, no kind of dialog box open.
It the activity monitor it shows 185.6 MB memory usage - out of 16GB, so this should not be the issue. CPU percentage is 4.9% - so probably not causing this either.
Restarted the game, the numbers are roughly the same, RAM is about 132 now