Efficient is the wrong word here, because the amount of computing work they can do for a given amount of electrical energy is pretty tiny compared with anything

Efficient is the wrong word here, because the amount of computing work they can do for a given amount of electrical energy is pretty tiny compared with anything modern. They're pretty light in terms of energy usage though, and because of the elegant simplicity of the system you don't notice how underpowered they are until you ask them to do something CPU intensive. (Think several minutes to draw fractals that even 20-year-old PCs can draw in realtime.)