Exactly this ^

In particular, under some ideal, unrealistic assumptions to simplify things, and denoting n the number of E-cores, E the efficiency and W_e the power consumption of each e-core, while respectively denoting m the number of each P-cores, P the efficiency and W_p the power consumption of each p-core, where E>P, we can calculate the express EFF of the cpu as

  EFF = (n*E*W_e + m*P*W_p)/(n*W_e + m*W_p)
      = (n*E*W_e - n*P*W_e + n*P*W_e + m*P*W_p)/(n*W_e + m*W_p)
      = (E-P)*n*W_e/(n*W_e + m*W_p) + P
      = (E-P)/(1 + (m/n)*(W_p/W_e)) + P

which shows that the energy efficiency of the CPU increases monotonically as a function of the ratio of e to p cores n/m.