First, I think it is important to ascertain whether what God commanded is the same practice currently done in American hospitals. fisheaters strongly argues
here and
here that what God commanded the Jews to do was radically different from what Americans do to their infants today. So I think it is important to distinguish between Biblical circumcision and modern American male genital mutilation. I think the pain is also substantially less, in proportion to the reduced amount of flesh being cut away.
As for pain, as C.S. Lewis observes in
The Problem of Pain, it can serve to point us out of ourselves towards a greater reality. This would explain pain adult converts would have experienced. As for infants who apparently lack cognitive resources to integrate it, perhaps their suffering is the consequence of sin corrupting our environment, as other things like natural disasters and allergic reactions: Perhaps sin results in an environment whereby humans must develop pain receptors at an earlier age than was God’s original plan. (We need more defense mechanisms in response to more attacks.)
You might also note that God deliberately causes women to suffer more pain in childbirth as a punishment for original sin – and, apparently, increases their reward proportionately for bearing children.