Without checking the implementation details one thing that looks problematic is that for Jsonifier library CMAKE_BUILD_TYPE=Release is passed explicitly whereas for simdjson it isn't. Similarly for the Header only ones (e.g. Glaze) -O3 is being passed

Alright I see what you mean. And as such I've opened this:
Loki-Astari/JsonBenchmark#30

Without checking the implementation details one thing that looks problematic is that for Jsonifier library CMAKE_BUILD_TYPE=Release is passed explicitly whereas for simdjson it isn't. Similarly for the Header only ones (e.g. Glaze) -O3 is being passed

Alright so it's been fixed. And the results are the same.