Upper bound for runtime complexity of LOOP programs 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)
Take Ack(n-1). Grows much slower than Ack(n).