In this paper, we introduced a new figure of merit, overdrive delay product Pod which is defined as the product of overdrive factor (I/Ic0 - 1) and delay of transit time, to evaluate power consumption and switching delay from the viewpoint of perpendicular magnetic tunnel junctions (p-MTJs) switching. The impact of sub-volume excitation on the dependence of overdrive delay product on the junction size and material parameters of p-MTJs in adiabatic regime were clarified. Two strategies to decrease the Pod were proposed. The first strategy is scaling down the junction size free from sub-volume effect. A reduction more than 86% of Pod of p-MTJ with exchange stiffness Aij = 19 pJ/m was realized by scaling down the junction size from 70 to 10nm when I/Ic0 - 1 = 0.5. The second strategy is to increase Aij to suppress the effect of sub-volume excitation. A 26% reduction of the overdrive delay product was realized by enlarging Aij from 10 to 31 pJ/m with annealing process in the p-MTJ with the diameter of 40 nm. These results indicate that p-MTJs of embedded magnetoresistive random access memory (MRAM) should be scaled down under 30 nm where no sub-volume effect occurs for high speed programing.