Agree entirely, I have flown through a fair collection of airports in the US, although not PHX to be fair, and immigration time has varied from more or less walking straight through to standing in a line for over an hour. It all depends on what is in front of you.

If you are the only plane at the time then you can walk through, the problem comes when you have aircraft in front of you and, unfortunately where those aircraft have come from. If they are from countries that don't speak great English (far east mainly) then it slows you down with forms filled in wrong etc. Same goes for any country homeland security isn't too fond of (not going to name names here in case of flame war erupting)!