To clarify the GIL issue (for davin, I guess? :): if the library you are using to interface with the FORTRAN code drops the GIL before calling the FORTRAN, then you *can* take advantage of multiple cores.  It is only the python code (and some of the code interacting with the python objects) that is limited to executing on one core at a time.  (As far as I know it isn't restricted to be the *same* core unless you set CPU affinity somehow, and I have no idea if it improves performance to use CPU affinity or not).