Yep, I think you'd be hard-pressed to find the actual Elixir code to be the bottleneck in a real-life application. But if you do encounter that, you can use something like Rustler[0] for the CPU-intensive bottleneck, as Discord did[1] while working on a data structure they needed. Slow DB queries are something else to look out for.

edit: Didn't see Rustler was already mentioned in a sibling thread, but hopefully the Discord tidbit will be interesting to others.

[0] https://github.com/rusterlium/rustler

[1] https://github.com/discord/sorted_set_nif