With call_stack-2.patch attribute access in namedtuple is only 25% slower than attribute access in ordinary Python object! Definitely this this worth to continue to experiment!

But adding new slot to PyTypeObject sets the bar too high. Try to use your function to speed up all cases mentioned in issue23507: sorted()/list.sort(), min() and max() with the key argument, filter(), map(), some iterators from itertools (groupby(), dropwhile(), takewhile(), accumulate(), filterfalse()), thin wrappers around special method (round(), math.floor(), etc). Use it in wrappers around PyObject_Call() like PyObject_CallFunctionObjArgs(). May be this will cause an effect even on some macrobenchmarks.