PSC motor speed affected by capacitor?
The new motor could be a suitable replacement but be designed for a different capacitor value. The capacitor value should be marked on the motor. However, the wrong capacitor value would probably not cause a big difference in operating speed. It seems more likely that the motor is wrong or that it is a two-speed motor that is connected for the slow speed.

A defective new capacitor or a capacitor that has a much lower capacitance like 1 or 2 MFD could cause the motor speed to be significantly low.