Problems with Penumbra BP running slowly

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I have reached the part of the game where I need to distract an 'infected' to get into the Chemical Lab but it is running sooooo... slowly it is impossible to play it. I have a GEforce 5200 graphics card with the latest driver. I have turned off any unnecessary features but it's made no difference. Any suggestions??

 

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Slowwwww

Besides the engine is "light", on certain places the game slow down to me too (Athlon x2 4800 - Radeon 3750) ...... to be more precise, when fog is near......