Ok but that's the point. You are just explaining why the 'protobuf is faster' claim is generally false.
The comparison that's being made is equivalent to implementing quick sort in Python and bubble sort in C++ and then declaring bubble sort is the better alternative.
Anyone with a healthy understading of computer science and programming experience will not make bullshit claims like that.
Which link were you comparing FFI in python to json in python? The link I see seems to be about pure JavaScript language Protobuf impls vs json.parse?