Rewrite in Rust: never :)

A non-Ruby frontend may be possible but, fundamentally, the Formula DSL is Ruby and we couldn't feasibly move to another language without breaking backwards compatibility with everything.

The "update tons of stuff you never asked for": we're working on this: https://github.com/Homebrew/brew/issues/19202

Performance in general: we're doing what we can to improve it wherever possible. Ruby makes this harder but it is getting better rather than worse at this point.