Usually I wait until evening to ride. But I've ridden a few warm humid days this spring -- temps and humidity in the 80s. Cyclemeter sez I'm riding about the same speed, but I know I'm taking it easier on hills and into the wind. I'm consciously trying to reduce my effort during midday rides so I'm not exhausted later.
Main concession I've had to make is to get some faux-chamois pad undershorts under my baggies, after developing a perineum rash. Cotton just doesn't cut it for riding in hot tub air.