Hello, armorer here. You’re not wrong. Well-fit armor should not slow you down or hamper your ability to run, jump, or fight…as long as you have built up the stamina necessary to do these things. Yes, armor is heavy, but the weight is distributed evenly across your body, so it does not weigh on you like an equal weight in a backpack.
Someone who is not wearing armor will be more nimble, of course, just because they don’t have to fight the additional inertia of the armor. But you are by no means clumsy or clomping. That’s Hollywood nonsense.
There are historical records of knights training in all kinds of ways to get used to moving in their armor. One knight in particular was well-known for putting a ladder against a wall, hanging underneath it, then climbing up the underside of the ladder using only his arms.