- It should speed up CPython for the standard benchmark suite.
I don't think point 5 is a goal here either, as the problem isn't that these calling optimisations don't exist, it's that they don't currently have a public API that third party projects can access (the most recent METH_FASTCALL thread covers that pretty well).
Agreed. The goal is not to speed up CPython but to bring third-party extensions up to speed (both literally and figuratively).
For clarify, main goal is not just only 3rd party extension faster. Publicate some private APIs is enough for it.
Goals of these PEP 576 (GitHub version) and 580 is making custom callable type (especially method-like object) faster.
Because most functions and methods are defined with PyMethodDef and m_methods / tp_methods, these PEPs are not needed for them.
I think main motivation of these PEPs are modern Python usages: Jupyter notebook + Cython.
Unlike extension module writer, we shouldn't expect user knows difference between C and Python. That's why Cython want emulate normal Python function/methods as possible.