Here’s a data point from ATL. Recently I came in to ATL with a friend - both US citizens. I have GE; he does not. We decided to compare Immigration processing times. At ATL, both GE & MPC have dedicated lanes. He got through faster in the MPC lane - primarily because that lane is in the middle of the Immigration hall, whereas the GE lane was on the far end (farthest from baggage claim) and I had a much longer walk to get there. Neither line was long.