To be fair, I don’t think that absolute peak performance is one of the goals of the Roc language. You can read what they mean by “fast” here:
I think the idea is with pure functions there are no mutating APIs – mutation isn’t even a user-visible property. That way you don’t need pointer-heavy persistent data structures, you can use normal ones (with good cache locality).
It’s explained in more detail on the “functional” page.