I can't attribute it to racing but while in the Corps there were a ton of times when we would run in 70 degree temps and then the the next week have to train in the 100's (either in the Gulf or 29 Palms) and yeah it does affect performance. For me cycling is a different animal since riding provides somewhat of a fanning effect. I really don't start moaning about the heat until it hits 107. Granted, I whine like a girl if its under 65.
For some temperature change is worse than others. Every time we hit the tropics on our deployments half the ship would get sick for a few days.