Message passing in Erlang will be a fair amount more expensive - Erlang and C++ have far, far more differences in terms of performance than just message passing

Message passing in Erlang will be a fair amount more expensive - Erlang and C++ have far, far more differences in terms of performance than just message passing vs shared memory. You're better off comparing C++ and Pony. At that point, a message enqueue is 3 atomic instructions, not too bad. The benefit is trivial concurrency.