My one OTTB and my clyde cross both do horribly in the heat. My clyde is worse due to body mass and size. With them I don't ride if it's high 80s and humid, let alone 90s.
I also have to be careful as I am prone to heat stroke thanks to having it as a kid. So I know if I start feeling lightheaded at all, or nauseous, that I need to stop and call it quits. Because this happens easily to me even when well- hydrated, it's a good monitor for when I need to call it quits for my horse's sake as well.