In this paper, a comparison is made of several proof calculi in terms of the lengths of shortest proofs for some given formula of first order predicate logic with function symbols. In particular, we address the question whether, given two calculi, any derivation in one of them can be simulated in the other in polynomial time. The analogous question for propositional logic has been intensively studied by various authors because of its implications for complexity theory. And it seems there has not been as much endeavour in this field in first order logic as there has been in propositional...