One thing I've noticed is that it adds in ambient occlusion, giving stuff a less washed out appearance. There might be other stuff to, but I don't know what else. In terms of fps, it's pretty minor (going from high to medium gives me like less than 5fps).
As far as I can tell, it makes the game look better, enables motion blur when on medium or higher, and will have an effect of about 5 fps in resource-intensive areas like London Victoria. Of course, that 5 fps difference is between low and medium (I like to use medium when possible because I am one of those weirdos that likes motion blur, but most of the time I am forced to keep it low in order to maintain an average of 20fps)
Oh great I should mention that the 5 fps is a difference, not a value It will not for from 20 to 5 fps, but instead it will go 20 to 15fps
