is the lens an early standard mount whose mount rotates inside the camera lens port? The focus ring is solidly connected to the protruding rear of the mount on those lenses, so that the whole outer bit rotates while the internal part with the key slot remains held by the key inside the camera port. With those lenses it was recommended to lightly lubricate the mount to facilitate the rotation. Of course there could be other reasons it feels stiff - contamination inside the camera port, a burr or out-of-round bump on the lens mount, damage to the groove at the very rear of the lens mount, or damage to the spring arms that lock in to that groove and hold the lens in. Or the lubrication of the lens internal helical thread could be dried up and gummy.
If it's a standard mount lens that doesn't rotate inside the camera lens port, then the stiffness will be due to internal lens lubrication.