I´ve been through immigration at IAH
many, many times, both for connections and with IAH
as a final destination. IMHO, 90 minutes is usually extremely close if you are a non-US citizen. The CBP
officers there really take their time and sometimes the new immigration facilities are rather understaffed.
If I were you I would sit as close to the front of the aircraft as possible, run up the stairs (as opposed to taking the escalator)...and pray.
In your case, what I think will be working in your advantage is your MEX
flight´s arrival time. If I´m not mistaken, the only other international flight coming in near that hour is a 767 from AMS
(can´t remember at exactly what time does it get into IAH
I´ve waited in line for passport control there for nearly an hour, being the first off the plane and before it gets really crowded there, so I can´t even begin to imagine what it must be like with 400-500 people ahead of you.
OTOH, since the new terminal was opened my luggage has been waiting for me at baggage claim and, save for once, I´ve really had no trouble going through security upstairs for my connecting flight.