I haven't sent to Mexico recently, but I know that First Class International can be slow. The key word is "CAN be". It can also be speedy . . . It just depends. I sent a First Class International package to London that took a month-and-a-half to arrive, yet another time I sent a First Class Int package to London and it arrived in three days. And the two packages were sent within a week of each other too.
Fingers crossed for you!