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".
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.
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.