Answer: First of all, check and make sure there are no air leaks in the line, and that your compressor is producing the recommended pressure: 17 CFM at 90 PSI.
If the air supply is adequate, check that the pneumatic tool is properly lubricated - it needs to be oiled to perform best.
When the tool is noticed to be running slower, add 1-2 drops of oil into the air inlet. Do not add too much oil.