Is there something wrong with the way this thing is connected?
You've created a &quot;source follower&quot; circuit with the motor connected between source and 0 volts. The maximum voltage you will see will be about 3 volts across your motor and, as a result, it is unlikely to spin very fast.
A much improved circuit is the &quot;common source&quot; circuit where the MOSFET source is connected to 0 volts and the motor is connected between drain and +12 volts. With the right MOSFET you'll be able to get over 11 volts onto the motor but, the motor has to be capable of working with a ground connection (most will of course).