New information: I think I have pinpointed at least a contributor to the difference - closure lookups seem to be currently slightly slower (by a few percent) than global lookups (see https://stackoverflow.com/a/46798876/541136). 

And as we can see, an inner function that references itself is a closure on itself (see LOAD_DEREF):

>>> def foo():
...     def bar():
...         return bar
...     return bar
...
>>> bar = foo()
>>> import dis
>>> dis.dis(bar)
  3           0 LOAD_DEREF               0 (bar)
              2 RETURN_VALUE

This, at least to me, explains why the performance difference doesn't completely amortize away.