Why not use Go? They have one of the best networking stacks among language standard libraries, excellent support for low latency soft real-time concurrent operations, and single binary deployment is trivial.
The best language for the task at hand, when presented with time constraints, is the one that you already know well. OP said in the article that they authored Papercups [1]. Adopting Elixir for a websocket-push service makes a lot of sense, then. However, why don't you learn Elixir, some OTP, and then reconsider that question? You could be missing out.