https://github.com/zserge/q, an experimental minimalistic clone of Vue, uses object.defineProperty. (There has been an attempt to use Proxy instead, but there's a problem with it: https://github.com/zserge/q/issues/1) I don't think I could make it any lighter if I wanted to.
If the value is remote, I assume a websocket or regular AJAX poll. If the value is local, simply binding an event emitter to the DOM should do it. Both would be lower load and compute than most frameworks.