If you double your exhaust velocity, you double your thrust but quadruple your power requirements. If you have a fixed amount of power, then you’ll have to cut

If you double your exhaust velocity, you double your thrust but quadruple your power requirements. If you have a fixed amount of power, then you’ll have to cut thrust by one half. Spacecraft are usually severely mass limited so less thrust and more energy consumption in exchange for better mass efficiency is usually the right tradeoff.