With the application of energy-maximizing control for wave energy converters (WECs), the WEC design problem becomes a control co-design problem. One of the fundamental requirements of co-design is to evaluate the optimal control performance, i.e., average power generation. Previous control techniques include model predictive control (MPC) and pseudo-spectral (PS) control, but both require iterative optimization, with computational requirements the main limiting factor in co-design. In this study, a fast optimal control performance evaluation method is proposed based on a ‘wave-by-wave’ (WbW) representation. The idea is to split the wave excitation force (WEF) signals into individual waves, process them separately, and then combine the results with the distribution of WEF amplitude and period, yielding a straightforward average power calculation. The method is fully developed and studied, considering the cases of position-only, and general, constraints, as well as different choices to obtain the WEF parameter distribution. It is shown that the WbW method can achieve a very high control evaluation fidelity (within a 5% error) and give almost the same co-design result as MPC and PS (implemented using WecOptTool), but with a significantly reduced computation time (e.g., hundreds of times faster), therefore being a game changer for control co-design of WECs.