Running in warm humid weather without comparing pace to cool-season numbers

My easy pace in cool weather is comfortable, but the same speed in warm humid air pushes my breathing and heart rate noticeably higher. I keep looking at the watch and feeling as if I'm getting slower. I know comparing those numbers directly is silly. Do you run by effort, heart rate or a deliberately slower pace during humid months? And how do you avoid turning every sticky run into a test of fitness?