Declaring your methods via the prototype is faster, but whether or not this is relevant is debatable.
If you have a performance bottleneck in your app it is unlikely to be this, unless you happen to be instantiating 10000+ objects on every step of some arbitrary animation, for example.
If performance is a serious concern, and you’d like to micro-optimise, then I would suggest declaring via prototype. Otherwise, just use the pattern that makes most sense to you.
_process()). Most developers will understand and avoid these properties, unless they’re willing to forgo the social contract, but in that case you might as well not cater to them. What I mean to say is that: you probably don’t really need true private variables…