Very low framerate, modest hardware

[spidernetlabs]

This happens to me as well and it's not only a Mac OS X issue; it happens on Windows too. They are working on a patch, so finger crossed.
You can try this, and see if it helps:
open main_settings.cfg (Library/Application Support/Frictional Games/Amnesia/Pig) with TextEdit or similar programs and change MaxChannels="64" to 48, StreamBuffers="8" to 6 and StreamBufferSize="262144" to 32768.