Initially, you will have neither enough carbohydrate nor ketones to fuel your brain. While you are always producing ketones, it takes time (up to 2-3 weeks) for your body to increase production to the point you are relying on them as a primary energy source. During this period, training performance will definitely suffer (and lifestyle performance may suffer as well). Your power output will be lower than normal. Your running pace will be slower than normal. Perceived exertion will go up, at all intensity levels. Recovery from training sessions will be hindered.