"Other than that, nothing prevents one from writing the performance sensitive part in C/C++/Rust/etc. and then using whatever other language for the non-perform

"Other than that, nothing prevents one from writing the performance sensitive part in C/C++/Rust/etc. and then using whatever other language for the non-performance sensitive portion, which typically represents the bulk of code." Whoops, didn't address this part. This would be a bad idea, since the "low performance" portion could still cause unpredictable GC pauses, affecting the "high performance" part. It also seems a bad idea to split the code base into two languages for no particular reason, when you could write the whole thing in a nice language like Swift. :^)