In general, an increase impeller speed in a stirred vessel operated in turbulent flow is expected to accelerate mixing, heat or mass transportation and reaction as plus factors. However, it also increases anxiety about the minus factors of higher power consumption of the impeller, damage of the impeller by fluid resistance or cavities behind the impeller blade and breakup of crystals. By considering both plus and minus factors together, namely, by adding a "human touch" that balances the degrees of expectation and anxiety, a method of determining the optimum impeller speed based on information entropy is discussed in the light of previous experimental results for versatile impellers, and a new method of determining optimum impeller speed is presented.