This study investigates the impact of separator porosity on the fast charging and discharging performance of lithium-ion batteries (LIBs). High-porosity and low-porosity polyethylene separators (HPPEs and LPPEs, respectively) were employed to check the influence of separator resistance on overall cell behavior. Electrochemical analyses revealed that LPPE, which has higher ionic resistance, had little effect on fast charging but markedly reduced discharge capacity under high-rate conditions. We demonstrate that this asymmetric behavior originates from the mismatch in diffusion coefficients between the cathode and anode, whereby the high resistance of the separator exacerbates the low diffusion coefficient of the cathode during fast discharging. To support this observations, four cells with bilayer separators were fabricated (cathode/HP–HP/anode, cathode/HP–LP/anode, cathode/LP–HP/anode, cathode/LP–LP/anode). Among them, the cathode/HP–LP/anode configuration exhibited better high-rate discharge capacity compared to the cathode/LP–HP/anode cell. These findings demonstrate that separator properties may either emerge as a limiting factor or remain masked, depending on the coupled effects of separator resistance and electrode diffusion. In conclusion, this study underscores the necessity of considering diffusion properties of active materials when optimizing separator design for high-power LIBs.