Why does freshwater diffuse faster than saltwater?

Salt water has, well, salt. As the water diffuses through the membrane, the concentration of the salt left behind increases(So long as the membrane is not permeable to salt, of course). The increased concentration makes the water want to return to the salt to dilute it. This makes the diffusion slower beause the water keeps backtracking to reduce the salt's concentration.

Read the principles of osmosis.

Fresh water is much less salty, and hence creates sharper gradient.