No Mac Issues? or No Mac Users?

[THELASTPOWERPC]

Hey, Im a Mac user with an issue. The game launches and runs, but the frame rate or movement is slow. I've played the demo until reaching the water level, and while using lower settings, but found no difference. Here are my specs and settings:

Powermac G5 2.3 Ghz dual core (late 2005)
Mac OS X 10.5.8
14 GB DDR2 SDRAM
Nvidia GeForce 6600 PCIe 256mb ram

Amnesia Settings
Quality:Medium
Resolution 1024x640
Fullscreen
Texture quality High
shadow quality medium
Shadow Resolution High