> You can have all those wrapped under a high level API, with much nicer implementations, with more support, more documentation, more eyeballs (due to more adoption) in a "slow" language.

You 'can' have those things, but 'should' you? Julia is well designed, so 'nicer' is not a given. Julia is also quite a high level language with excellent metaprogramming facilities. Otherwise, everything you listed is based on adoption level.

Why use a "slow" language if you don't have to? Especially when the faster language is actually better designed...