Framerate issues with Amnesia: The Dark Descent

[AdamAK]

I did a file integrity check, but the game has been re-installed several times over the past few months anyway (during my attempts to fix the problem).

I've managed to make the game playable (the low fps kicks in later, and is mainly an issue in larger rooms) while streaming, but it's still underperforming substantially compared to how it used to run.