The JSON encoder uses a lambda function for the sort(key=...) invocation used to sort the keys in a JSON object in case sort_keys=True is passed:

https://hg.python.org/cpython/file/46bfddb14cbe/Lib/json/encoder.py#l352

Instead of having a lambda, operator.itemgetter(0) can be used here, which is a minor performance gain. It should be noted that many other internals of the JSON encoder have also been fine-tuned (manually) for performance reasons.