O.K., A 1 HP electric motor running at 1750 RPMs produces 3 Ft. Lbs. of torque. So....a 1/2 HP motor ( which is what most industrial sewing machine motors are) running at 1750 RPMs produces 1.5 Ft. Lbs of torque. A clutch motor is either on or off, just like the clutch in a car. An experienced sewer, or an experienced driver, can make a car, or in this case a clutch motor, slip through the foot pedal (or clutch pedal) to make it a variable speed device, but this takes a lot of experience and it is still either on or off. The slower you go the more torque you can produce. Servo motors are constant torque devices, so at any speed, a 1/2 HP motor will produce 1.5 ft. lbs. of torque. A clutch motor running full speed will produce the same amount of rated HP and thus the same amount of torque, and perform exactly the same as a servo motor. But: clutch motors on sewing machines rarely, if ever, are run at full speed, so they are more powerful at slower motor speeds than servo motors.
What does this mean to a sewing machine operator? Although my sewing machines have less power than a traditional clutch motor at slower speeds, the accuracy and control I have over the machines more than outweighs any loss in power. I have been sewing for almost 35years, and I have yet to find a situation that the servos won't handle. I wholeheartedly recommend changing clutch motors to servo motors. If you can't afford to change to a servo motor, put an extremely smaller pulley on the clutch motor to slow the machine down. If you put a pulley on the clutch motor half the size of what came on the motor, the speed will be cut in half.
I sew every day for a living, and have a Consew 255 RB-3 and a Consew 226- R that I switched to servo motors. I have not noticed the tiniest bit of reduction in performance in either of the two machines.
BTW, sewing speed is not a factor in quality, .......ever.
After looking through my post, I realize that, once again, I may have over-answered the original question. If that is the case, I apologize.