I have run in the Gulf(very salty) and also in Lake Tarpon(fresh) on the same day and noticed NO difference.
Sure, saltwater is more buoyant because it's more dense, but at the same time the hydrodynamic drag coefficient is also more due to this increased density.
Most of the claims of incresed speed are due to altitude. Saltwater is always at sea level and most lakes are above.
Atmospheric pressure decreases roughly 0.5psi for every 1000' of elevation. See here:
Elavation this matters