#1 is just FUD. JavaScript is JavaScript. Class in JS is a shorthand for function and functions prototypically delegate. That you're not writing in Java/Python/C#/Ruby should be pretty obvious.

#2 doesn't bear much relationship to reality. There will be near zero correlation between "class" and performance. What can (and today, does) change the performance of objects is having their shape change markedly after construction. V8, for instance, already includes heuristics to know when a class is "done" being constructed, allowing it to apply most of the typed-language tricks in the bag for objects which are entirely unfrozen. In current code, it's actually a de-optimizer to call Object.freeze() and there's no reason to think you'll get anything faster for using "class" than by writing code which doesn't change its shape very much.

You will, however, finally be able to read and share code more easily.