My particular problem was a back end for a medium-level multiplayer RPG (500ish people connected at once per world). The only real way I found to get it performant was to lean heavily into processes. But then you have a race condition problem and need to DIY your own method of synchronization because you're no longer using the process mailbox for that purpose.

In practice I haven't found the immutability of Elixir that useful for concurrency because data is deep copied when it goes across processes anyways. This is in comparison to Clojure where you can safely and perfomantly share immutable data across multiple threads.