Using multiprocessing adds some overhead for because of serialization, so it's slower than it would be to just hand off the Python objects directly to the workers (as you can with threads) because the process doing the parsing also has to spend time on serializing them again. So you can avoid the GIL, but it has a cost.

For example, if I parse an XML document with ElementTree, as a quick experiment, parsing the document takes ~1 second and serializing all the elements names and attributes to JSON takes an additional ~0.5 seconds. Serializing the whole ElementTree object using pickle takes ~4 seconds. Serializing it as XML takes roughly as long as parsing it.