Very low framerate, modest hardware

[Bergeton]

Hi, I bought A Machine for Pigs on GOG and I'm having severe performance problems.

My computer:
Macbook Air (late 2010)
2.13 GHz Core 2 Duo
4 GB 1067 MHz DDR3 RAM
Nvidia Geforce 320M (256 MB)
OS X 10.8.4

I can play Amnesia The Dark Descent perfectly smooth with high/medium settings
(High textures, medium shadows, paralax on, 8x anistropy, shader effects on).
A Machine for Pigs, however, runs, to be blunt, like Bantha poodoo.
No matter how low I set the graphics options, the framerate is so, oh so, slow.
-Unless I'm right at the edge of a map, and staring at a wall, then it runs great.

I know A Machine For Pigs is supposed to be slightly more demanding than The Dark Descent but this can't be right, surely?

Anyone else experiencing performance issues in OS X?
Can anyone think of any solutions?
(don't you dare say Bootcamp, if I wanted to play in Windows I would )

Any help is greatly appreciated