I'm having the same issue, and then some. The extreme slowdown only lasted for about 2 dialogue lines when I got to the town (although those two lines took quite a while). It seemed fine, but returned when I got the mine key from the molezard and exited the mine behind the blacksmith, although not quite as severe, but still unplayable. Going back in the mine, the slowdown isn't there, but returns as soon as I exit again.
Saved, reinstalled, reloaded, issue persists. Never played or installed the demo, so I doubt that's relevant.