Our bodies have to work harder to keep themselves cool while we run on a hot day. It diverts energy that we would otherwise be able to use to run faster.

Most heat adaptation is in the form of your body sweating more, depending on how drenched you are normally, that may not help too much. As everyone else has said, going by HR is probably your best bet but if you're looking for some magic, I would doubly make sure you're hydrated before going out (I measure my specific gravity by pissing on a strip) firstly and drink as close to your sweat rate as possible (probably unlikely on a run).

You can also try pre-loading on a high sodium solution (similar to Osmo preload or Skratch hyper hydration mix) before heading out to give you a hydration buffer. I've used those a few times and always feel quite good for the first hour or so in hot/humid conditions. Finally, if you top your glycogen stores up the day(s) before an intense workout in the heat it should help since glycogen has a ratio 3:1 water:carb (ie. as you burn up the glycogen, it'll release water into your system).

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I like analyzing things - http://engineeringfitness.org

Soaking buff tubular at aid stations assists a bit when temperatures get high.Extremely hot event locations always have a hose running somewhere,again this is just a brief fix but find helps with moral.Watching the tour de france i came across the stockings filed with ice technique, yet to try this but it seems like a efficient way to cool down if ice is available on race day.