How to slow down an 12V electromotor/fan
There are multiple ways of slowing it down, which would all result in the fan drawing less power. 3 options I can think of


Add a potentiometer in series with the power cable. This would allow you to change the speed quite easily. This is the simplest method but in terms of power quite wasteful because you lose power over the potentiometer. 
control the fan using pwm. This could be with a microcontroller or just a 555 oscillator through a transistor. This adds a bit of complexity but is more efficient powerwise. 
Have a constant current circuit. This would be done with analog electronics, also more power efficient.