If you're interested in performance, what about all the number conversion (to decimals, presumably) that is incurred with JSON?

If I'm interested in performance I'll build my data out of offset handles and lay everything down into a block and mmap() it around. That's parsing free, up to an htons() — but that's only a worst case scenario. Everything else is about not inventing something custom & being able to use easily vendored high-trust 3rd party tools. (In this case: a JSON library, LZ4, and/or SQLite.)