Here's an interesting article about heat vs. speed
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2039810/ that I just found and read (part of).
A couple of examples are contained, the first is a 30 minute time trial in 73F and again at 90F. I assume this is in well conditioned athletes. The difference in this example was a reduction of 6.5% in power, which translated to 2.8% slower. This is an interesting subject that I never thought much about other than I just try not to ride when it gets really hot.
The article mentioned the possibility of why power is reduced could be because of mitochondrial leakage. I don't know what that means, but it sounds horrifying.
