Why is a 12V 3-wire CPU fan very slow at 12V input?
I figured it out myself.

There is some speed control built into the fans board. I can see a small part coming out from the side of the stator case. Might be a temperature sensor. I pointed a heat gun at it from behind the air flow. The fan revived to significant speed :-) 

Shorting the leads of the termistor (youtube.com/watch?v=AVovy7UWDQ0, is exactly the same fan by the way) lets the fan run at full speed and I can control RPM now by lowering the voltage.