We have done May...but have also done August pretty much every year since 2009. May is much more pleasant, but that doesn't make August unbearable.
I pretty much always give the same advice, but it's true: go to the Parks at rope drop. Either have lunch there, then leave for the resort pool, or have lunch at the pool. Enjoy your resort, the pool...the pool activities, etc., and come back to the Parks around or after dinnertime. It will be less crowded and once the sun goes down, less hot.
Wear what works in heat and humidity! Some prefer that dry-fit fabric, we find we do better in light colored cottons. My DD's and I honestly think sundresses with slip shorts underneath are far cooler than anything else. Sandals (the good supportive kind) work much better than socks and sneakers. Drink lots of water, slow down, and take air conditioning breaks often. We love our August trips! We can stay for 10 nights with no one missing school or busy times at work, and the Parks don't seem that crowded to us.
I will say, we had to go in July of 2016 instead of August, and I found it much worse! I was very happy to return to August this summer! Enjoy!