If a non-generic type derives both PartialOrd and Ord, the PartialOrd impl can just do this:

impl PartialOrd for T {
    fn_partial_cmp(&self, other: &T) {
        Some(self.cmp(other))
    }
}

(It won't work for generic types because the trait bounds on the generic parameters won't be right.)

This will likely be faster to compiler than a full implementation. It might also be a little easier for the backend to optimize.

Some futher discussion is on Zulip here.