It is commonly accepted that different structural parameters of electrode materials (i.e. S_BET, S_micro, V_tot, R_t and I_D/I_G) have different effects on the electrochemical properties (i.e. specific capacitance, power density, energy density, and rate capability) in terms of the influencing magnitude and trend. It's therefore extremely desirable to establish a relatively unified structure–function relationship using a single performance indicator. This work luckily finds a defined/composite performance indicator (S_micro/S_BET), which can uniformly influence the capacitor performances. Such analysis is based on the characterization of the as-assembled symmetric electrochemical double-layer capacitors using a series of samples as electrodes (obtained by a dual-activator strategy with C₃N₃Na₃S₃ and KOH as dual activators and egg yolk as precursor). All the electrochemical performances (the specific capacitance, rate capability, energy storage performance, etc.) have the same tendency with S_micro/S_BET changes and the peak for all the performances can be obtained when the S_micro/S_BET value is 53.53% in this research. Herein, S_micro/S_BET is expected to be one composite performance indicator to reflect the structure–function relationship and in turn judge the whole performances of the materials through the relationship of S_micro/S_BET and a single performance, thus greatly simplifying the screening of electrode materials for high-performance supercapacitors.