Membrane solute permeability plays a role in the buildup of concentration polarization in pressure-driven crossflow filtration processes, and thus in the determination of the permeate flux, solute rejection, retentate flux and concentration. We numerically examine reverse-osmosis desalination with membranes of fixed solvent permeability, but of variable selectivity with respect to the solute. The study highlights an intricate coupling between retentate and filtrate properties. In particular, it reveals that, for given values of solute permeability and feed concentration, there is a maximum operating pressure that optimizes solute rejection regardless of the feed salinity. The conditions leading to this and to other peculiar behaviors for permeation fluxes and concentrations are identified.