I have seen a pop up pandemic in the last several days. Maybe you have also noticed with your own running?
Symptoms: Loss of confidence, sudden loss of fitness, slower running, especially at harder efforts.
A brief summary is that the following dew points result in the corresponding drops in performance.
60° = 2%, 65° = 3%, 70° = 5%, 75° = 12%, 80° = 15%. Note the accelerating decline as it gets warmer. Remember these are dew points and not simple air temperatures.
Add the rule of thumb that you will slow down by 2% for every 1% in body weight loss due to dehydration and you have a losing battle. There are several other factors including acclimatization (generally takes 2 solid weeks) and individual heat tolerance.
Let's say the dew point is 75° and you have lost 3% of your body weight in sweat loss. If you would normally run 7:30 pace or 450 seconds per mile, the total of 18% loss of speed (12% for 75° dew point and 6% for the 3% dehydration loss) you could expect to slow down by 81 seconds per mile. So now instead of cruising along at 7:30's, you are now feeling lousy and running 8:41 pace. It's physiology. You might be tough and might tolerate heat better than most, but you will be working much harder to even be remotely close to your normal pace.
Solution: Consistent training for adaptation and adjusted expectations. Stay hydrated using salt tabs when necessary. Use respiratory rate and perceived exertion as your primary measures of effort. Pace and HR will be affected to a degree that their reliability is no longer as helpful. Continue to rely on Run Smart, Run Hard. Heat will require you to emphasize the smart, the hard just happens.