Does shadow boxing make you faster?

At best, shadowboxing with weights is only a conditioning exercise. It can strengthen your shoulders and that extra strength in itself may give you a little more power and speed.

What is the advantage of shadow boxing?

Shadowboxing is a great way to build up that much-needed muscle memory. While you're in control of your surroundings and you're focusing on your form, technique, and movement, your building these important skills into your muscle memory to be able to move about the ring easily and comfortably.