Fortran isn't really used much for down-to-the-metal optimisation. It's not at all built for the low-level tricks common in C, but rather expects you to program at a higher level (notably with first-class arrays), after which the compiler will (hopefully) do its magic. Although in practice, most Fortran code is not particularly optimised (or optimised for long-dead machines), and could be much much faster.

So, if you want to do down-to-the-metal optimisation for Fortran, you want to write a Fortran compiler, not a Fortran program.