Not sure if your theory can really account for the 7X difference between Play and Netty in the JSON test. They both create a trivial name/value pair and pass it to the JSON serializer. In Play's case, it gets passed to their Jerkson wrapper for Jackson. What would you even change about that code?

(Note: I'm not necessarily saying the Jerkson wrapper is the culprit. Could be Play's routing framework, or something else.)