The words 'Factors A, B & C in autorotation' popped unbidden into my head on reading the last few posts - at least a few things must have stuck over the years.
Haven't thought to look back over that stuff for a good while, but looking back at the aerody books shows a few factors in play when increasing forward speed from zero in autorotation and comparing rates of descent.
As my aerody manual would have you believe, factors A and B relate to disc tilt and act to increase rate of descent as speed is increased, whereas factor C is about the disc flying into undisturbed air and acts to decrease rate of descent.
As published in flight manuals, and as we know from experience, there are best (lowest ROD) speeds for autorotation, so what Crab says sounds right to me.