DuraGRIT® rotary tools are rated to run up to the following speeds:

High Speed Rotary Carbide Cutting Wheels & Carving Burrs
(Model # starts with SR- and have 1/8" diameter shaft)
Run up to 35,000 rpm

Industrial Carbide Tools
(Model # starts with LR- and have 1/4" diameter shaft)
Run up to 15,000 rpm

DuraGRIT® tools have maximum speed ratings. Like any tool however, you should start slowly using light pressure and increase the speed until you reach the optimum cutting/shaping performance for the material you are working on.  

With wood that would typically be when the wood starts get hot, smell or burn. At that point, reduce the speed slightly.  

With plastics, increase speed gradually - again using light pressure - until the material starts to heat up or melt and then back off slightly.  

At all times - DO NOT apply too much pressure.  

Let the tool do the work and you will achieve a nice smooth finish and at the same time reduce the need to clean clogs from the cutting surface of the tool.