For the most part, I prefer to see the arms of my whirly spinning as fast as possible.
Ive noticed that when a battery is dying and the machine begins to slow down, incoming pigeons actually take notice of the movement of the arms much more and start to become hesitant about coming close.
Funnily enough I am currently field testing a model which comes with a remote speed controller which allows me to change the speed from the hide.
I dont envisage too many situations where slowing the machine down will be advantageous, but there could be instances where speeding it up might help.
Needless to say I will keep you posted about my findings.