A crossbow is fairly easy to learn compared to a bow, but can take time to master.
Being a master bowman has a lot more in common with being a master swordsman, in that your body IS the weapon, the tool is a lever and striking edge/point allowing you to accomplish something you couldn't do with your body alone. So a lot of the training is getting your body into better shape and condition, because your physical capabilities are the limits of the weapon's abilities.
With a crossbow, or gun, for the first shot at least, you're providing a steady firing point, aim, target selection, and choice of the moment to fire. . .but unless you're too feeble to aim steadily or have a twitch, your physical capabilities have a limited part in the result.
Learning how to shoot once is a task you must master for these weapons, but generally you end up spending more time learning to re-load quickly and cleanly, how to maintain the weapon, and how to carry it around without damaging it, then you do on the firing line. (Especially with smooth bore firearms where loads of aiming skill was far less important than getting your gun and powder to the field ready for use, and your skill at re-loading under non ideal conditions.)