Cool. This might be getting into the weeds a bit, but are you still on rustc-serialize for json or are you trying to keep up with serde/serde_json? If you're using serde, are you on nightly? From your comment above I got the impression that only using stable features was very important, so I'm curious how your codebase implements/derives the serde traits.
We're on rustc-serialize. JSON is not really a part of our data pipeline, just our metrics pipeline. So the performance of the library is not especially critical.
