As per the instructions you can preheat on high for 30 - 60 minutes and then adjust the temperature to suit selecting 1 - 9.
As the throw rugs are usually used in the open on a lap the ambient temperature may affect the performance.
For example, a traditional electric blanket for the bed has additional layers over it and the heat is insulated within the top layer.
You can place an additional blanket over the throw rug to help insulate and hold in the heat.