Because their entire purpose is to be there fastest, it's in the epithet "fastest man alive."
Superman is strong, usually the strongest among Earth's heroes but he's hardly the strongest character in DC outside when the narrative calls for him to find that eleventh hour reservoir of strength to go that extra mile.
He's my favorite character, but I don't think he has to be the absolute best at everything. If he truly is going to be the Apex of strength alongside J'onn, he absolutely needs to be slower than the Flash. Frankly I don't see why this matters so much to you. Superman has always been as strong as the story called for. Bendis recently had him averting disasters and taking care of a robbery in Gotham whilst holding conversations. He's not exactly a chump.