first-the servos must be the same. second the load on the servos must be the same (work). if all of this is the same then swap the servos mechanically and see if the speed moves with the servo. if it does then take the slow one apart and lube it minimally. if all of that is not helping then run the slow servo in by repetitive cycling the slow servo. if it gets slower (it shouldn’t) then run the other in with repetitive motion.