Granny toss the medicine ball. Its a hip hinge movement, that doesn't really work "slow". To make the ball further/ farther the movement will have to be better timed and more coordinated.
Use the inverted row. To make the move "easier" she'll have to develop full body tightness. Also, she'll try to go faster to make it easier. But if she still goes slow, it makes the move more effective. It's win-win.
Is she moving slow because the weights are too low? If you go a little heavier, will she be forced to move faster?
Or is she moving slowly because the moves are too challenging? Do you need to regress to less challenging stuff for her?