why is that?

Sadly in Python using string functions is sometimes more performant than calculating these values.

From my point of view (I'm someone who is just starting to learn Python), the most important thing when designing a function wouldn't be efficiency, the time it takes to execute, right? Whenever I manage to solve a function, I compare it with others using the timeit module, and if one runs faster than another, it's better. Many times, I find that some written in just one line are not the most efficient.