As I guess you're hinting at, writing performant pure Julia is a quite different kettle of fish to writing performant C: it's not surprising that a first attemp

As I guess you're hinting at, writing performant pure Julia is a quite different kettle of fish to writing performant C: it's not surprising that a first attempt isn't a rousing success. But there's a spectrum of rewriting possibilities: you can write Julia-interfaced-with-C as a direct analog of the existing code, and you can convert just those parts of the C code that you think would most benefit from Julia's JIT into Julia, leaving most of the heavy lifting in C.