I’ve taken a look and while I don’t have the time to dedicate to a PR I can offer the following observations (which may already be totally obvious to you).

1. Since the :only and :except lists need to be known at compile time you can process them in the macro, outside the quote block. That will certainly give you some performance improvement and make it easier to maintain as well.

2. Most encoding tasks that result in a binary (string) gain material performance benefits in encoding first to iodata and then calling :erlang.iodata_to_binary/1 at the very end. That function uses very efficient kernel system calls to convert io vectors to binaries. File.write/2 and friends support iodata as a parameter do you don’t even have to do the last call yourself and in effect the File.write/2 call will stream the iodata to disc (not exactly how it works, but close enough).

Those are the two primary differences I see with the Jason implementation being very idiomatic macros techniques.

Since that narrows the scope I’ll take a first pass at a PR for point (1) tomorrow my time. Don’t expect a finished product but at least enough for you to move forward. Hopefully gets you on your way at least.

Guess it’s reading the book then.