These high temperatures affect comfort, safety and speed. Instead of just reading humidity and guessing how it will feel to run outside, record the dew point (in degrees F) and the current temperature (in degrees F) and add them together.
This will give you an idea of whether your run/race will suck or not:
Temp+dewpoint of 90-100 is probably ideal, but anything below 130 is pretty much OK to go for the fast time…maybe 120 for longer distances…say 1/2 marathon or above. Between 130-150 will see a gradual slowdown in pace, and if it’s above 150, forget pace altogether and just focus on finishing.