True at least to PY2.7 and 3.5 - given x is a numpy array, say np.random.rand(int(1e6)), then sum(x) is much slower (for 1e6 elements - 2 orders of magnitude) than x.sum(). 
Now, while this is understandable behaviour, I wander how hard it is to add a condition that if argument is a Numpy object then use its own sum. 
I think many programmers aren't aware of that, so all in all it can improve the performance of a lot of existing code.