The problem here is the ambiguity of "safe", I think. Saying rust generates "guaranteed safe code" seems to imply that compiled rust can never do anything wrong, which isn't true.

Rust does make guarantees about certain kinds of safety, when not using unsafe {} blocks. You get memory safety and freedom from data-race issues. I don't know if any other languages offer that without incurring runtime costs.

Slow rust code is also possible, but your code can't really be slow because it is written in rust, in the same way that c doesn't get in the way of performance.