[QuadrupleA]

It's more about reducing complexity than the build time I'd say. I work in C++ too, and a couple seconds of compile and run isn't the end of the world.

I don't personally have to deal with browser differences because I stick to subsets of CSS / JS that are 98%+ supported. If all your polyfills just compile down to minimal compatible JS anyway, then maybe those language features aren't that crucial. Always try to solve the problem without the new feature - almost every time I've tried this I find I get a simpler, more maintainable result without it, e.g. simple callbacks instead of promises (sacrilege), var instead of const/let (your poor namespaces), the word “function" instead of an arrow, etc. Those are probably close enough to supported to start using now, though not sure I care. Template literals might be nice, sounds like support is close.

Re: typescript, even coming from the C world, I don't personally find type safety to be a deal breaker. I work on some large python codebases - dynamic typing makes a lot of things much easier and more concise, in exchange for not catching some errors during compile. It's a tradeoff.

Avoiding all this build nonsense does make it harder to use some libraries that come out of the crazy modern node / javascript world, but in general most good ones have release builds, and I find a bit of healthy distance from the technical tail-chasing of that ecosystem is good for productivity.