Manufacturers are often over-optimistic when describing their products.
Shredding paper creates quite a lot of dust that tends to clog up the works of the machine. They start off ok but soon start to slow down and lose power that can be restored by cleaning and lubricating the mechanism with something like a silicone spray or a dry lubricant.
The older the machine is, the more often it will need servicing to maintain peak performance. It is possible the thermal breaker in your machine is a little tired and very likely it isn't receiving enough cooling air to the motor - perhaps some vents need clearing or modifying.
