Just FYI the example that person gave would absolutely not explain a huge performance difference. I don't think they understand what they're looking at.

fair enough :D but it does happen and there are reasons for that: https://stackoverflow.com/questions/47363996/why-does-an-enum-transpile-into-a-function

This isn't true, there are other features that "emit code", that includes: namespaces, decorators and some cases even async / await (when targeting ES5 or ES6).