Bandit is written in pure Elixir which is a bit undesirable to integrate with pure Erlang projects. From what I gleaned off the podcast, it's a subset of the HTTP features available in Cowboy, specifically those that are available on the Phoenix side, and cleaving off the unused functionality realized much of the performance benefit.

Bandit author here. Correct! The byline of Bandit is ‘a web server for Plug applications’ and being able to focus on that narrowed set of requirements is a large part of where the perf boost comes from (less code, easier to reason about, fewer processes, etc).