Motor speed based on uF of capacitor
Two capacitors in series is half the capacitance. Try three in series or one 0.5 uF capacitor. Reducing the capacitance reduces the available torque and allows the load to slow the motor down. At some point, the motor may not have enough torque to start reliably or may vary a lot in speed while running. At that point, you will know you are trying to reduce the speed more than the motor will tolerate.