Apparently that isn't always the case. See the PyPy Status Blog: PyPy's new JSON parser https://morepypy.blogspot.com/2019/10/pypys-new-json-parser.... which talks about being more efficient with both deserialization and memory.

It's a long time since I looked, but profiling my code not much time was spent parsing / serializing JSON. Most of the time spent was manipulating dicts/lists in Python which cPython is already pretty good at since the whole language seems to basically be implemented in terms of dicsts. I don't think PyPy has the hidden class optimizations of JS engines which are able to find speedups in these types of cases.