Here in W. central Georgia, we've been solidly in the 90's, with heat indexes pushing 100, and I've been doing 3.5 hour road rides, nearly every day, with two climbs over our resident "mountain". I try to be on the road by 9 a.m. but today it was after 10:00 ( but i only did 2 hours, cuz my quads were feeling puny today, and I have stuff to do later.)
I used to race and train in Cozumel,Mexico, a climate even more oppressive than the steamy south, and I only recall a single day where I was thinking, "Good lord, this is just too hot !!!!" (it was a crit-like race up and down the length of the airport road, in the afternoon in August IIRC).
Mostly, I've always felt comfortable riding in very hot weather. It's the cold that zaps my will to be out there !!!