No, that's it - for most json documents, the time the interpreter spends in all the stuff surrounding (and executing) json.loads dominates the time spent in nat

No, that's it - for most json documents, the time the interpreter spends in all the stuff surrounding (and executing) json.loads dominates the time spent in native code parsing. That's why replacing json.loads with a faster native implementation doesn't result in noticeable improvements - it's speeding up the fastest bit, and if it's already 0.1% of the wall clock time, then you can't get much more than about 0.1% faster. PyPy is a JITed language though, so the interpreted bits are much faster. OP is making the point that Python 3/CPython is so slow that speeding up json.loads is a premature (and unnecessary) optimization.