Dissolving Tylenol Science Experiment - why is it fastest in distilled water?
Water can only dissolve a certain amount of solids at a given temperature before it reaches saturation. Distilled water has been boiled and captured in gas phase, almost completely eliminating the presence of dissolved solids.  This lack increases the number of ' water to solid' molecular interactions that will take place and so speeds up the disolving.  The same would be true of any solute.