CoffeeScript is largely an alternate syntax to JS with a couple of macros added on. That's more like the prima ballerina changing from her pajamas into a tutu.

Indeed, they don't go as far as some of the other ones, but it's still a language that compiles down to JS. It's nice that it's a strict superset of JS (iirc anyway) but that alone doesn't mean that it's impossible to produce slow or bloated JS.

Of course any language that does a lot more than CS is going to be slower, e.g. Objective-J's object system providing method-missing functionality and a whack of other features not found natively in JS.

Coffeescript I'm familiar with and yes, is very nice indeed. It's some of the others I'd be more concerned about.

If we can enable source pointers for debugging, then "good" matters less -- about as much as how "good" x86 ISA is.