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.