Went to Mazatlan and Monterrey, it was HOT, low or no AC in every building except where we stayed. The fridges in every store were not that cold so it was hard to find an ice cold beverage (I don't really trust the safety of their ice/water). The masks made everything that much hotter. Even the outdoor shopping malls required a mask in Monterrey. It had been about a decade since since I last visited Mexico and I don't remember the heat being like this.
This trip really has me second guessing my heat tolerance. I'm starting to think October, and not September, would be best for Southern Europe.