So, what computer is kind of second-best? Please do not ask me just not to turn off the power, because energy is never free.
Otherwise, if you really want vim and emacs, you might consider trying to put something together that can be always on. S3 sleep is low latency, and can be lowish power. Or get an early x86 chromebook and boot to freedos; chromebook because the firmware is small and doesn't do much; x86 because software compatability; early because the earlier ones are easier to run 3rd party firmware on for better use of non chrome OS; freedos because it's small and fast to start (assuming it supports the chromebook keyboard anyway).
Early computers did turn on almost instantly but program load times from tape or disk was slow. Although in some instances I could say from direct experience that a c64 power on to text editor can be faster than a modern computer.
I believe back in the 80's the was some single purpose computers built with word processing, spreadsheet and other very basic office programs built in. These would be very fast but might not meet your idea of an OS.
Old systems that had cartridges, if you could find a text editor, also should be very fast. Again the idea of what you mean by OS may not be met.
Surely, the right tool for the write job does not make a Luddite.