Water usage absolutely. Speed to chill nope, that's the myth. I think you knew that though.
For those who don't: the water will match the temperature of the wort on the other side of the copper in short order thanks to the copper's conductance. The limiting factor in cooling rate is the heat capacity of the wort relative to that of plain ol water. Something we just have to deal with.
The technical options for optimizing are really two: agitate the water/wort such that the water/wort on the inside of the flow gets exchanged with the water/wort on the copper wall of the tube (hence convoluted CFC's and whirlpool arms) or, increase your surface area (bigger chillers). Both are still limited by the heat capacity of the water (once you match the temps, nothing's gonna change from that distance on down the chiller) and each has their design trade offs.
The goal of either design is to make sure that all the water (across flow's cross section) at the exit of the chiller is the same temperature as the wort (either in the kettle in the case of immersion or at the outlet in the case of CFC's and plates) Technically, that's as good as it gets. You can of course chill the water but that's not a chiller design issue, that's execution.
"In this house we obey the laws of thermodynamics!"
I would recommend bending up a whirlpool arm if you have a wort pump, it works very nice. I used to run it for the last 15 minutes of the boil to sanitize the lines then turn on the hose at flame out. Presently I am using a convoluted CFC and have a tangential inlet on my kettle to whirlpool cold wort back in and chill the whole volume as fast as possible.
Originally Posted by MalFet
I'm not sure what exactly you're saying is a myth here, but certainly slowing down the flow rate can make you more efficient with regards to water usage.