We had a MID(Merida)-IAH-ORD connection on a Sunday morning a few months ago, with no checked luggage. First, it took some time just to get to the gate (E8) because of traffic on the taxiway, then the US citizens/residents immigration line took just under 30 minutes. We had a two-hour connection and were fine, but some others in our traveling party had shorter connections and missed them.
That was the last straw for us, by the way--now registered for Global Entry.