I've also certainly noticed a decrease in my limit of exertion on hot days in recent years. Fortunately, we generally don't get too much of that. Some of it is my own fault for being under-hydrated at times. I generally don't like to drink too much since with my general prostate issues it can lead to a lot of urgent gotta-find-a-restroom moments along the way. But this year I am being more conscientious about it and trying to stay ahead of my thirst. I'm also taking in more in the way of electrolytes and maybe an energy bar or two on a long ride.