The director response times of pure IPS and VA modes were compared by inserting respective material constant values of practical LC mixtures into the equations based upon the Leslie-Ericksen theory. The VA mode was confirmed to be superior to the IPS mode from this aspect due to the difference of the Frank elastic constants for bend and twist. Further superiority was shown quantitatively because of the contribution of the back-flow effect in the VA mode. These results were supported by referring to the molecular theory of the physical properties of LC substances.