Last summer I planned to take off work a few hours early on a Friday afternoon and extend my normal 15-mile one-way commute home. Unfortunately that Friday coincided with one of the hottest days of the year. I swore I saw my Garmin display a temp of 118, but I think when I uploaded the data, it showed a high temp of "only" 112. Fortunately, as Marcus_Ti points out, it was a dry heat. As I recall, I ended up cutting the ride short, and ended up with about 30 miles for the afternoon (plus the 15 from the morning commute).
As long as I have enough water, the (dry) heat doesn't bother me too much. It may slow me down, but then, I don't ride that fast to begin with