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Saving the steps of writing and then rereading C code is a big reason why g77 should be able to compile code much faster than using f2c in conjunction with the equivalent invocation of gcc.
However, due to g77's youth, lots of self-checking is still being performed. As a result, this improvement is as yet unrealized (though the potential seems to be there for quite a big speedup in the future). It is possible that, as of version 0.5.18, g77 is noticeably faster compiling many Fortran source files than using f2c in conjunction with gcc.