One major difference you'll run into here is support for encoding protocols. Jason, Poison, and Jsonrs (by default) will have protocol impls for Elixir structs

One major difference you'll run into here is support for encoding protocols. Jason, Poison, and Jsonrs (by default) will have protocol impls for Elixir structs that define special logic for how to serialize those structs into JSON strings. ie, you probably want a DateTime struct to serialize as a date string rather than a map containing the year, month, etc. Jiffy doesn't support any of that, so you end up with differing behavior between it and other libraries as soon as you start encoding structs that aren't meant to serialize as maps. Jsonrs supports encoding protocols by default, but lets you turn it off for a speed boost if you're okay with more Jiffy-like behavior. Plugging it in as Phoenix's JSON library won't turn off that protocol handling, so you're getting the slower operational mode of Jsonrs (which is fine and probably what you want in most cases for correctness, but will definitely eat a few ops/sec in a benchmark)