When working in Python, it's best to avoid creating your own function if there's already a built-in one available for the task. Built-in functions like sum, max, filter, and map are highly efficient as they are implemented in C. Above is the comparison between the speed of using a built-in sum and the speed of using a custom sum. There can be a significant impact on performance when working with a large array, even with small differences in speed. For a comprehensive list of Python's built-in functions, you can refer to this resource - list