Shaded Pole motor - change RPM by relative change to windings
A shaded pole motor is a type of induction motor, just using some neat magnetic tricks to generate a second phase. It runs at slightly below synchronous speed, the slip depends on the torque.

For a simple 2 pole motor, you'd expect it to run at perhaps 90% of 3000 rpm (50 Hz countries), or 3600 rpm (60 Hz). This is what you are seeing.

If you reduce the voltage, with a series inductor or resistor, the motor will produce less power and depending on the load, the speed will drop and the slip increase. However the torque drops very quickly at reduced speeds, so it's unlikely you will get anywhere near as low as 50% of synchronous speed.

I think that removing turns, keeping the voltage constant, will increase the current and make it run slightly faster against the same load.

This site has some diagrams and specifications. The speed when running below rated speed will be very dependent on the torque curve of the load.

So unfortunately you can only reliably reduce the speed of the motor by reducing the AC supply frequency (and voltage) with a variable speed drive.