Here, we develop a "fast water model" that fully retains all energetic and thermodynamic properties but substantially increases the sampling efficiency. We combine the two approaches of (1) quantifying the stability of MD time integration and (2) mass repartitioning with (3) the fact that in dilute (aqueous) solution, larger-scale molecular motions and thus sampling efficiency are determined by the viscosity of the solvent medium. To speed up the solute dynamics, we repartition and rescale the masses of water to reduce the viscosity without causing time integration instabilities. A simple analytical relation for the rate of catastrophic crashes k crash in MD simulations as a function of the time step allows us to quantify the stability of the time integration. The resulting "fast water" is then used with a time step as before, to keep the time integration of the vibrational motions of, say, protein solutes stable. However, the reduced water viscosity and faster diffusion result in proportionally faster sampling of the larger-scale motions in the conformation space of both solute and solvent. We illustrate this approach by developing the TIP3P-F model based on the modified version of popular TIP3P model used with the CHARMM force field, but the scheme can easily be applied to other water models and even different solvents.