Rails is optimal for the web-tier which is traditionally scaled by being stateless, so you have one process per core (or thread if you are using JRuby) and you are done. It's ironic you list Node as the path forward since it is also single-threaded. Obviously it is more efficient on the baseline with async IO, but it still requires the same number of processes to saturate multiple cores, and you can also trivially eat into the IO overhead in ruby with green threads via Puma.