The reason the current C implementation pairwise is slower than the python recipe seems to be the usage of Py_TuplePack (as was already hinted at by @Stefan2 in the linked discussion on pairwise and zip).
Replacing Py_TuplePack (which uses varargs) with a direct Py_TuplePack2 makes the pairwise C implementation as fast as the pure python recipe. Branch: Comparing python:main...eendebakpt:putuple_pack2 · python/cpython · GitHub
The Py_TuplePack with arguments 1 and 2 is used several times in the Python codebase, so perhaps we can add Py_TuplePack1 and Py_TuplePack2 to the python API. I will create an issue to discuss that a bit later.
Update: issue is at Add Py_TuplePack2 and Py_TuplePack1 · Issue #118222 · python/cpython · GitHub