Why would the implementation language matter for the compiler? It is a traditional input-output algorithm, it is either good and does many fancy optimizations for good output, or it isn’t. Sure, the speed of compilation may vary, but that’s a different question.

Also, JS being such a dynamic language, it will likely perform better with a speculative JIT compiler, e.g. a shape can be assumed for a frequently used object. This is only possible with PGO to a degree.

In theory it could be: compiling for the specific latency profile and instruction-set of each individual machine. In practice that has never been done.