Using a dull bur for cavity preparation will result in:

Cutting effectiveness is the rate of tooth structure removal (millimeters per min or milligrams per second ). Effectiveness does not consider potential side effects such as heat or noise. Cutting efficiency is the percentage of energy actually producing the cutting. Cutting efficiency is reduced when energy is wasted as heat or noise.  It is possible to increase effectiveness while decreasing efficiency. A dull bur may be made to cut faster than a sharp bur by applying a greater pressure, but experience indicates that this results in a great increase in heat production and reduced efficiency. Ref: Sturdevant's art and science of operative dentistry. 4th edition page 340