str type still has a few methods using METH_VARARGS (slower than METH_FASTCALL):

* count()
* find()
* index()
* rfind()
* rindex()
* startswith()
* endswith()
* format()

Maybe I will propose a change later to convert these methods to Argument Clinic, but I consider that the initial issue "Currently, Argument Clinic doesn't use METH_FASTCALL for these methods, but METH_VARARGS" is now fixed, so I close the issue.

Thanks Naoki and Serhiy.