How efficient is Python's max function
It's O(n), since it must check every element. If you want better performance for max, you can use the heapq module. However, you have to negate each value, since heapq provides a min heap. Inserting an element into a heap is O(log n).
Of course it is O(n) unless you are using a different datastructure supporting the max of a value collection due to some implementation invariant.
It depends on how you are using it.If you want to maximize based on a function "someFunc", it'll take O(len(l)*k) where k is the time function "someFunc" takes to run.
maxVal = max(l, key=somefunc)But yes for normal case it should just iterate over the list and find the max using normal compare function.