"Melt" might have been August's most impressive attack bar Ars Magia, but you're forgetting to account Natsu's immense heat resistance + the fact that he managed to negate some of the damage. Also, Gildarts tanked this:
Whilst that is admittedly an unnamed attack, there's hardly been any named attacks as of late. Even Zeref was just throwing around nameless blasts until his fire attack, so there's no reason to doubt that this blast would be powerful in it's own right.
As you stated earlier, Cana tagged August because he was caught off-guard. As for August's speed/reactions, why would Racer's magic help him against Jellal + CS in a defensive way? Racer's magic doesn't slow speed, just reactions, so while I get it helping offensively (easier to crack their defenses), why would it help defensively?
And as for Gildarts' not being consistently faster, when did he try and blitz August again? And tbh, that doesn't even matter, because that one showing is better than any speed feat of Natsu's IIRC (though admittedly I'm not that knowledgeable on Natsu).
In their previous sparring sessions, Gildarts was never bloodlusted like he is here, and no opponent ever challenged him to the extent that August did. Why wouldn't he abuse his speed and agility, especially against a heavy hitter like Natsu?