Will computer fan rotate slower if one makes impeller heavier? Imagine that we replace plastic fan impeller with identical metal impeller (same form, heavier weight). Would maximum fan speed decrease?
Common sense suggests that air resistance and bearing friction do not change, but intuitively I would expect fan maximum speed to decrease.
EDIT: As I know, usually computer fans use brushless motors if that matters
 A: If the bearings were to be considered frictionless, then the maximum speed of the fan will not decrease, though it will take the fan longer to reach the maximum speed. 
Because as the moment of inertia of the impeller increases its angular acc. will decrease (for the same torque applied), therefore it will take the fan longer to reach its maximum speed. The maximum speed is when the power delivered by the shaft to the impeller will equal the power dissipated through wind resistance, heat etc.
A: This will depend on exactly what kind of motor you have.
If your fan is a brushed DC motor, then the fan speed will be slightly lower, since the new impeller is heavier than the old, so there will be slightly greater bearing friction. The added friction will serve as a power loss, and the motor will have to run slightly slower.
If the motor is a brushless motor, the speed is determined by the drive frequency, and as long as the bearing load is not too great the fan speed will remain unchanged.
A: Yes, the rotation speed will decrease.
The reason is that the fan gets a constant power (energy per second) from the system which translates to a constant torque that rotate the fan against air resistance (and such). Greater mass density of the fan means greater value of the relevant Tensor of Inertia component of the fan. This means it takes greater force to give it the same amount of angular momentum. On the other way around - for the same amount of force (which can be translated to power mentioned before) it will spin slower.
