Assuming you're happy with a C++ implementation of the parser, I've not had any issues with nlohmann/json; although there are faster libraries it has been fast enough for my requirements and has the advantages that:
- It is very easy to integrate into your project (single header)
- A clean and simple API without excessively verbose syntax
- Good test coverage
The Github page linked above has a great overview so it's not worth me going into much more detail here.
For a comparison of libraries along multiple facets (not just performance) you could look here: miloyip/nativejson-benchmark
