All I can say is...wow!! We went through on Friday night, and were overnighting. From the moment we deplaned to exiting the airport, took us 1h20, and that is the fastest it could have went. No X on our passport control paper. The lines at the luggage carousels were huge. So, having carry on only like we did, did not help a whole lot, as you still need to go through that line.
MIA as a passport mobile app, if we ever go through MIA again, might try that, as it looks at it would speeds things up.
Weird thing, we were about 25 people in the no X line, and multiple hundreds in the passport control X line....