The "two language problem" in the high performance numerical computation domain, in which you have a high level language that is easy to use as a glue (such as Python) and a low level language for anything that requires performance (such as C++). In Julia a program that looks like python can run at the speed of C (and as a bonus has the metaprogramming abilities of a Lisp minus s-expressions), and with only one high level language for everything you can much more easily create, understand, extend and debug anything.
But if you really want to write something that's fast you won't write it in Python, Julia, or these days - C. You would probably use C++ (in a non-C-ish, non-OOPish way), and in some cases (think DBMSes) you'd craft your own LLVM IR and have it JITed.
Why not just use Julia + a few crafty LLVM calls to get to the same place if you want to go that deep? That's essentially what you'd be doing from C++ there.
