Recently I learned about [LOOP programs](https://en.wikipedia.org/wiki/LOOP_(programming_language)), which always terminate and have the same computational power as primitive recursive functions.
Furthermore primitve recursive functions can (as far as I understood) compute anything that isn't growing faster than $Ack(n)$.

Is this implying that the upper bound runtime complexity for LOOP programs is $O(Ack(n))$? And are there functions similar to Ackermann's function, which can't be computed by primitive recursive functions, but grow slower than $Ack(n)$?

(sorry for spelling and grammar)