Let's go with 85/95. That's horrible, but it happens certain times of the day, certain times of the year, in certain parts of the country.
There are two effects.
The first is from the difficulty breathing. It typically isn't a factor in sprints or middle distance. But by the end of mile 2, you can tell you're breathing is in trouble. After mile 3 or 4, the full effect is felt, and the cost is real. I don't know the math, but from personal experience and from running with or coaching maybe a hundred others, I'd say it's 10% or more of a performance drop.
The second effect is from fluid loss or overheating. If you are a non-sweater, or an extremely light sweater, you can overheat. That might take 60+ minutes, depending on how hard you're going. If you are a heavy sweater, you probably won't overheat but sweat will pour off you like you're a garden hose. Your sweat rate can be so high that you can't replace it by drinking. You can't digest enough, quickly enough. With the massive drop in hydration will come a massive drop in performance, apart from breathing troubles. If your workout is under two hours, that might be ok, but if you push into hour 3, trouble...
The good news: If you train reasonably in high temp/high humidity, your body will adapt, and the adaptations will help tremendously in those conditions and in better conditions.
Some runners are figuring out that training in high temp/high humidity is as beneficial as training at altitude. Maybe more.