You don't combat it. You just adapt to it a bit more. You will always be slower. Just the way it is.
Drink more during the day, keep your house warmer (less AC), less AC in the car, etc. After 2-3 weeks there's a marked improvement, but I still register around 5-10% power loss in longer efforts most of the time according to my cursory analysis when the dew point is 70 and above.