I've found that using Git libraries directly is usually slow and less ergonomic. I've got another tool (written in Go) here: https://github.com/zikani03/git-monorepo which uses a Git library and is a bit slow.
But I'm willing to take up that challenge and test out gix, I've regained my interest in Rust so the timing is good :)
Actually doing this in Rust with lower-level libraries like gix would have been interesting.