From my understanding the things to check for slow sewing featherweights.
- Check the belt tension.
- Check the brushes, and commutator for deposits
- Check that the bobbin is disengaged.
- Check the foot controller.
According to
FW Doctor
If your motor is running slow and just doesn't have any zip anymore, loosen
the clutch to disengage the needle and step on the gas. Run it at top speed
for a long time until you hear the motor change its sound. Then its run
itself clean. It might take 10 mins, but you will hear the change in the
motor's sound when it's finally cleaned.