In [1]: def f():
   ...:     return 1
   ...:

In [2]: %timeit f()
73.7 ns ± 1.16 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [3]: def f():
   ...:     def g():
   ...:         return 1
   ...:     return g()
   ...:

In [4]: %timeit f()
217 ns ± 8.76 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [5]: def f():
   ...:     class C:
   ...:         def f(self):
   ...:             return 1
   ...:     return C().f()
   ...:

In [6]: %timeit f()
8.39 µs ± 94.1 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)