If you swing too hard, the computer will conclude the pendulum is lots longer than it really is. For little swings, when the mouse swings out twice as far, gravity pushes back twice as hard. But if you swing too hard, all that stuff isn't accurate anymore. Swing out twice as far, and gravity pushes back less than twice as hard. That makes things complicated.
Equations get hard to solve, and swings take longer to complete.
So the computer thinks the mouse cord must be longer than it really is. Back to the Swinging Mouse .