The single largest contributor to performance degradation on websites is that very industry.

Look JSON has it's advantages and is a fine tradeoff, performance isn't a place it's good at, that's not something that makes it a bad format, it's just if you want to optimise for performance I would start by reducing the sheer amount of redundant data being passed around in a JSON blob long before I would hyper optimise on a C++ JSON parser.

Sure if you are using a JS or python json parser there are massive gains to be had by calling into a lower level language to do the parsing but picking between the choices in C++ parsers is probably bikeshedding.

Now if your use case truely needs to absolute most performant JSON parser and you will trade off usability and portability for it then sure but another one of hose axioms apply. The solution for the 99th percentile is rarely the correct solution for the 50th percentile