It's all inter-related. Defogging/defrosting takes heat-energy, which is at a bit of a premium in the Prius. Because the engine runs intermittently, and is very efficient, there's not a lot of surplus heat to work with. The other alternative; electric heat, robs the car of it's fuel-economy, so there had to be a trade-off, which is reflected in the slower response of defogging/defrosting, particularly in colder weather. I still think the best solution might have been a gasoline-powered heater, as in some Volkswagens, but that would be too scary for some people to handle.
