That is completly untrue. The optimum speed is determined by the tip speed, a multiblade prop has the same tip speed at the same RPM and diameter as a two bladed prop. The efficiency does go down, but not enough to be noticeable. People assume this because there are not enough multiblade prop sizes to match their best two bladed prop performance. On high speed planes multiblade props can actually improve performance. Some WW II fighters went from three blades to four for this reason. But, this is not a factor on our models.