Does the electrical load on a generator affect its mechanical resistance?
Do you own a car? It's a very easy experiment, just turn everything electric on and you can "feel" the engine working harder.

The reason for this is the magnetic field every current creates.
Under no load you have the spinning magnetic field from the rotor, which creates a voltage in the stator.

If there is a load a current starts to flow which creates a magnetic field in the reverse direction. This hinders the rotors movement, so you have to put in more mechanical power to keep the rpm constant.