100F + will wear me down pretty quickly. So will dew points over 75F or so. I've commuted in both, in Southern California and the Gulf Coast of Alabama respectively. Once the dew points get up around 80F, I've got to ride very slowly to avoid feeling barftastic.
On hot days, I'll usually ride in the evenings - or do a shorter, flatter, and slower than usual daytime ride. How my body handles it depends on a wide variety of factors - pre-hydration, what I ate/drank the day before, whether I did something strenuous the previous two days, and even my mental state (tired from work, energized, angry, etc.).
A lot of the guys and gals swear by morning rides, but I'm fairly certain that the dewpoints are even higher than in the evenings (despite the temperatures being lower.) That's my excuse, anyhow. 5AM is not my thing.