So I was just hacking today around trying to introduce the ability to Base64 encode binary data within JSON for ThreeJS.
The performance of the final result is actually pretty good, many times faster than JSON.Parse for arrays of floating point data. Here is the data:
http://jsperf.com/json-vs-base64
The ThreeJS pull request to adopt this into the core library is here:
https://github.com/mrdoob/three.js/pull/3420
I think the applicability of this for speeding up JSON is quite wide, much wider than just ThreeJS.