The encoder module might be slightly better, because that is minified (less characters == faster execution).
If you can keep a steady RPM, and gradually increase the speed, you can figure out a rough limit of Espruino's setWatch processing speed.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@AkosLukacs no, but if I check source code of this module I'm doing pretty much same thing.