Framerate issues with Amnesia: The Dark Descent

[AdamAK]

then you are trying to load a save game made with an older version of the game

Ah, I thought this implied that loading old saves somehow messes the game up 'permanently'. In any case, I first played under an old save file briefly, and then started a new game. The game started lagging within roughly 15 minutes.

Any further ideas on what I could still try?