I experience the same issue, even after I'm fully heat acclimated.

Extreme heat will take even the most fit athletes down a huge notch just because the body has to work so much harder to disperse heat.

I've just resigned myself to the fact that I MUST take it easier in 90+ heat indexes than I would if it were cooler; I have to force myself to ride easier, take more breaks, and pay *much* more attention to my consumption of water, electrolytes, and potassium levels to avoid the effects of heat illness. Even then my best efforts aren’t always enough on some of my longer rides.

If you can find a way to keep your core temp down you’ll feel much better, but that is difficult to do on the bike. Ziplock bags that you can refill with ice in your bra and shorts, cool water on the head and neck, step into an air conditioned rest stop like a gas station, etc, in order to recover a bit are a few suggestions that have worked for me. The trick is not to do these things when you are already feeling the onset of heat illness, be proactive and try to anticipate it.