Nauli speed: Like the waves of a river?

I once read that Nauli should be performed at the same speed that waves lap up on a riverbank.  Does this ring a bell for anyone?  I can't remember where I read this, one of the older texts I think…but I just checked my copies of HYP and GhS and they aren't that specific.  (HYP just says “with speed” in my translation) 

1) Does anyone know where this passage comes from?

2) Does anyone have an idea of how fast this is meant to be?

3) How fast do ya'll do Nauli?  My average pace is rather speedy, about 2 revolutions per second, but some of my colleagues here prefer to perform it much slower, say 1 revolution per two seconds or so.  The quality of energy that gets stirred up seems a bit different depending on the pace, but I don't noticing it being hugely beneficial one way or the other.  Any comments?

Thanks,

Andrew