datetime handling can absolutely be a hot spot, especially if you're parsing or formatting them. Even for relatively simple things like "parse a huge csv file with dates into dataclasses".

In particular, default implementation of datetime in cpython is a C module (with a fallback to pure python one) https://github.com/python/cpython/blob/main/Modules/_datetim...

Not saying it's necessarily justified in case of this library, but if they want to compete with stdlib datetime in terms of performance, some parts will need to be compiled.