> Can the PR be applied then? It looks good to me.

Go for it Serhiy. Even if it isn't faster today, we might find more optimizations later, at least in term of C stack consumption.

Moreover, I understand perfectly the cost of having to parse arguments. Checking for keywords in the caller seems reasonable.

This change breaks the backward compatibility with Python 3.6, right? I mean, code using METH_FASTCALL. I guess that only Cython extensions use it in the wild, and I expect that Cython extensions don't use the stable ABI but are recompiled for each 3.x release, so it should be fine in practice.