This is best explained by a 2011 article found in Runner’s World. At the end of the story dew point is incorporated into the explanation, which most similar articles tend to overlook. http://www.runnersworld.com/race-training/training-heat
To briefly summarize, the closer the dew point is to the air temperature, the more saturated the air is and the less perspiration can evaporate and help the body cool itself, resulting in extra stress on the heart and lungs as the body attempts unsuccessfully to cool itself. Accordingly, the dew point provides a strong indicator of how you’ll feel during competition and a useful tool in predicting how much performance will be impacted.