Rotator cuff weaknesses do not translate into fatigue and low reaction speed. You might be confusing terms.
If you feel your arms react slowly and your shoulders feel fatigued too early, you are talking about the big muscles that raise and move your arms: rear, lateral and front deltoid in your shoulders, lats and other back muscles and so on. Those big muscles pull the humerus so that it changes its angle in space.
The head of the humerus is not perfectly spherical, but it is a complex surface with areas of different curvature radii. For the arm movement to take place correctly, simultaneously to the pull exerted by the big muscles there is a fine mechanism in action that puts the humeral head in exactly the right position, so that the pull from the big muscles does not result in a displacement of the bone or an impingement of soft tissues, but rather a smooth turn around a curvature center.
That group of small muscles deep inside your shoulders and back, not noticeables from the outside, are the rotator cuff. You never feel them (fatigued or whatever) unless they are injured. The less undesirable symptom you feel is usually that your shoulders make popping noises, but usually the result of weak or injured cuff is soft tissues impingement that causes pain and tendon injuries in the upper part of the shoulder.
The picture is simplified (the cuff muscles are known to assist in part of the range of motion when you move your arms in certain directions) but I think it is enough to give an idea.
You are probably looking for shoulder strengthening exercises. Dumbbell shoulder press with natural grip (palms facing each other) and elbows pointing slightly bent forwards (the so-called scapular plane) are quite safe as long as you start with very light weights and build up slowly. Avoid behind-the-neck presses at all cost.
If you are young and your shoulders don't hurt and you haven't noticed strange popping noises when you move them, you probably don't need any additional work for the rotator cuff. But if you insist on looking for rotator cuff exercises around the internet, please be careful. Those internal and external rotations with 90 deg elbows that you will find everywhere are meant to powerfully isolate the small muscles of the cuff in an artificial way, and it is extremely easy that you do a lot of damage to your shoulders if you don't do them properly.
The right way to do that internal and external rotations (provided that you insist in giving a separate workout to your rotator cuff) is using very light resistance and high rep sets.