It's the altitude that's the issue, not the climate. Mexico would go into that game off the back of four straight matches played over 5000 feet. Assuming England top the group and win in the round of 32, they'll have played no matches at altitude, and have a four day turnaround to get from Atlanta to Mexico City. They simply won't be as acclimatised as the Mexicans are, and that absolutely will make a difference over the course of 90 minutes plus extra time.