I'm not sure how high of a priority the speed of writing out JSON is. But if it is, I have a prototype that runs in about 40% the time of the fastest library I know of, which is https://github.com/ulfjack/ryu (to be fair, ryu tries to cover a larger set of use cases than we need). In many ways it's just an inverse of what we do for string to double, and uses the same multiplication trick.