Recently I learned about LOOP programs, 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)