This is how electrical motors operate: they accelerate very fast from a stop due to having high torque at low speeds, then as they catch on to speed they're operated at constant power until they reach nominal rpm. In order to achieve greater speeds, you need to operate them in the flux weakening region. E-bikes normally have engine cutoff at 25 kph so you're basically on your own legs over that threshold. No need to operate an e-bike drive with flux weakening.