Pretty fast, I didn't time it but it heated hot tap water to the target temp in a few minutes.
I can play around with the temp a little bit if you want to know.
You thinking a copper coil in a water bath to adjust the temp through heat transfer instead of direct contact, correct?
Right. If the heating is quick and accurate then step mash would be pretty easy without one of those massive electric brewing controllers. Plus you can use it for sous-vide.