Discussion: Should functions in lowpp layer be `def` rather than `cpdef`?

Functions in the lowpp layer are currently declared with cpdef, so they are exposed to both Cython and Python. However, we have always claimed that the Cython interface is not stable, and shouldn't be relied on.

A couple discoveries today are making me wonder whether we want to keep exposing the lowpp layer to Cython at all:

• Calling a cpdef function from Python has an additional 50ns of overhead vs. a def function (the Python layer delegates to an underlying layer and there is additional work done at that boundary).
• cpdef functions can not have their return type annotated, which is a blocker for proper .pyi support for the lowpp layer.

Is Cython access at the lowpp layer providing any value here? Could we just s/cpdef/def/g and get more performance and a more Pythonic experience?

Inizia individuando le funzioni nel livello lowpp e verificando come vengono esposte a Cython e Python. Confronta il comportamento cpdef esistente con i vincoli indicati relativi a .pyi e alle prestazioni, quindi controlla la discussione per verificare se è stata presa una decisione. Il lavoro è completato quando il progetto ha concordato se mantenere o meno l’esposizione a Cython; l’issue non identifica file o test specifici.