I haven´t much experience about the framerate, because I never had any big project...
But it seems as if the post-processing is very fast. In my test I just did, the framerate drops of about 3-4 fps from 50 to 46.
The Sky should be faster than the 3dgs skysytem but it´s even less flexible.
The water is slow but I´m shure, that it could be much faster.