Metro 2033 is one of those games that taxes your computer because these eastern block developers are crazy and like to render each water droplet and calculate dispersion, reflection, light refraction, shadow, condensation, physics, humidity, etc.
It's like physx in Mafia 2. Turning it on tanks the game's frame rate because it adds physics calculations to clothing of every NPC. By going into the files and moving things around, you can make it so it only affects your character's clothing (in addition to the other effects like tire smoke, debris) and it's perfectly fine.