The limits of Python vectorization as a performance technique
Vectorization in Python, as implemented by NumPy, can give you faster operations by using fast, low-level code to operate on bulk data. And Pandas builds on NumPy to provide similarly fast functionality. But vectorization isn’t a magic bullet that will solve all your problems: sometimes it will come at the cost of higher memory usage, sometimes the operation you need isn’t supported, and sometimes it’s just not relevant.
Source: The limits of Python vectorization as a performance technique, an article by Itamar Turner-Trauring.