Well when NaCl is placed in water it dissociates to Na+ and Cl-.
What happens then is the oxygen in the water molecules are attracted to, but does not bond to, the Na+.
Since the O in water is partially negative.
With Cl-, the H's in water surround the Cl-, since the H's have a partial positive.
So if it is indeed the effects that the Na+ and Ca+2 that affect the rate, then it could be seen that the Na+ and Ca+2 have a harder time effecting the rate since they are hindered my the attraction of the water molecules.
And water does not affect the rate as much with the salt in solution since it's partially positive H's are hindered by the attraction to Cl-.
So for distilled water, it may be that the partial positives on the H's have a similar affect as the Na+ and Ca+2, but more effective
Anyone have any other comments???