The first time I flew into MEX I also experienced an hour-plus wait in line. However, other times the process has gone very quickly. I really have no idea what goes on in that airport...

(And it's not just with immigration. Once I was flying out of MEX in January, and in the passage that leads from the big duty-free store to the US airlines' gates, I swear they had the a/c on. I could see my breath in the terminal!)