The basic classes int, dict, list, tuple ... all have a fast path for Py_*Check(obj):

#define PyLong_Check(op) \
        PyType_FastSubclass(Py_TYPE(op), Py_TPFLAGS_LONG_SUBCLASS)


except for float. I propose to add a Py_TPFLAGS_FLOAT_SUBCLASS enum and use it to implement PyType_FastSubclass for float.