There is fast and to fast. To fast is when the operator is wasting time picking up a load he lost or spending money on damaged loads.
As for your lift I presume it is a propane lift. You first have to make sure that the engine in performing to spec. It should have a fast idle of 2500/2600 rpm and drop 200/300 rpm in tilt relief. If the engine does not performe on the hydrolics it will never drive fast. If it does ok on the hydrolics then you may have a torque convertor or a trany problem.
If the spec of your lift is 10 mph it should go that fast or do you want to go faster then the spec's ???