And I would say that 3-4 PM in the summer is about the hottest time of day. I am very heat intolerant and I don't go in the summer, but even when I go in April or September (generally the hottest months I can tolerate Disney), I go LATE in the afternoon--we will go to the parks for dinner at 4:30 or 5:00, and by the time we get out of the restaurant at 5:30-6:30, it's cooled off a bit.
I'm also not a morning person so we don't go to the parks very early at any time of year (maybe 9:30-10:00 AM at the earliest), but if you're going during hot weather, very early or very late (before 10:30 AM and after 6:00 PM) are, to me, the only tolerable times.