Weird, I quit playing it after a couple of minutes because it was getting really laggy at the entrance of the Coal Pits on an i7 gen 8 processor. Unplayable laggy.
I once hit an unplayable slowdown (on Windows, though), where the devs told me I somehow managed to break the physics (NaNs everywhere). May have been a similar issue, as otherwise minor slowdowns are usually accompanied by excessive things happening (and unless you have an auto-cast Nuke wand, they're infrequent).
The game is incredibly CPU sensitive. Ensure that nothing else is using your CPU.