Many, many people who follow fasting/ketogenic diets report heightened energy and focus, myself included. This usually comes after the carb/keto flu phase once adjusted to the low carb intake and after the body has gone fully into ketosis. You have to be careful to get enough salt while fasting or eating very low-carb, though, as your body can struggle to hold on to water without those carbs you're used to eating. Consuming too little salt can definitely contribute to feeling sluggish.