Using a cython binding compared to the Ctypes one gives a speedup of a factor of 3. That's still not very fast, now putting the whole thing into a cython program. Like so:

    def extern from "newplus/plus.h":
        cpdef int plusone(int x)

    cdef extern from "newplus/plus.h":
        cpdef long long current_timestamp()


    def run(int count):
        cdef int start 
        cdef int out 
        cdef int x = 0
        start = current_timestamp()
        while x < count:
            x = plusone(x)
        out = current_timestamp() - start
        return out

Actually yields 597 compared to the pure c program yielding 838.