Elli: Performance oriented web server written in Erlang 14 years ago
Cowboy developer here. A good part of that is the lack of the Date header. The other main difference is the lack of routing (that you are going to do in any real-world application anyway). Also depends on your Cowboy version, the one that uses Ranch (which I'm about to push) got an increase in performance due to the removal of a bottleneck.