Cycling in the heat can be rough. Here in NC it gets hot AND humid, that can be a tough combination. It can slow you down a lot during the hottest parts of the day. I try to let the sun get lower in the sky and ride in the evenings. I like to watch the sunsets when I ride too.
The mornings are probably the best in the summer, still cool from the night before, most of the time, but I am lazy and don't like to get up early.
You also have to be careful because you can get heat stroke too. You have to stay hydrated and be careful not to get too hot. If it is really hot it is not pleasant to stop because of no breeze and the heat coming up off the road. Narrow roads with lots of trees close to it help a lot in the summer.
I need to add that most of the time the only breeze in the summer is when there is a thunderstorm coming.