First of all, check and make sure there are no air leaks in the line, and that your compressor is producing 17 CFM at 90 PSI, the recommended pressure.
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. Only lubricate pneumatic tools.
Read more: When and how to oil a pneumatic sander.