Why is a+b faster than a.__add__(b)?

Question

Python

>>> from timeit import timeit
>>> timeit('''
... a = 10
... b = 5
... c = a + b
... ''')
0.24442141791507765
>>> timeit('''
... a = 10
... b = 5
... c = a.__add__(b)
... ''')
0.5271954476593237
>>>