EV aggregators participating in frequency regulation services with a large number of EVs may have a delayed response problem due to their underlying communication infrastructure and scheduling. In this paper, we investigate the effect of delays in EV aggregators’ domain on their frequency regulation payments, especially mileage payment. Since the delays affect frequency regulation performance, in order to consider the delay effect on regulation performance, we enhance the conventional load frequency control model by including multiple EV aggregators with varying delays. Under this model, we evaluate the effect of delays in EV aggregators’ domain on their profits under two ISOs’ mileage payments such as PJM and NYISO. Through simulations, it is shown that the profit of EV aggregators increases as the delays increases. The conventional mileage payments did not motivate EV aggregators to reduce the delay in order to improve regulation performance. To solve the problem, we propose a new mileage payment to reduce the delay. We finally show that the proposed payment can provide an incentive to reduce the delay.