First, the StaticArrays package is already pure Julia code.
Second, StaticArrays aren’t a panacea — as I pointed out recently in another thread, they are only good for problems involving lots of tiny arrays whose size is fixed at compile-time, and aren’t a good idea as a general-purpose container.
Third, one of the key design goals of Julia is that code does not have to be “built-in” to the core language to be fast — for the most part, the standard library has no advantage over external packages except being installed automatically. In particular, there is no performance advantage to being “built in”.