Superman doesn't just think faster, he's also more knowledgeable. He's more like a computer in this regard. Batman, OTOH, is better at deductive and panoramic reasoning: at making connections and drawing not-so-obvious conclusions, including what-if scenarios.
Shinier than a speeding bullet.
Superman's intelligence has varied from being smart to super-genius through-out his history in various continuities.
If you're talking Post-CRISIS/Pre-FLASHPOINT, he was always at least very smart(if not genius level), but a number of instances had him as a super-genius, especially when he got an official intelligence upgrade under Kurt Busiek Post-INFINITE CRISIS, in the UP, UP AND AWAY storyline. His brain simply remembered and processed far better.
There are a number of instances after this that showed Superman as smarter than Batman, from improving on technology Batman made to Batman being jealous of Superman's improved intellect.
But most of the time in JLA appearances, they're going to have to dumb down everybody to make Batman more useful. I cringe reading when Batman actually had to come-up with the "brilliant" tactic of having Superman and Wonder Woman tackle General Eling(in Shaggy Man's body) at super sonic speed, like neither of them could think of that on their own and they needed Batman to create that tactic.
__________________ Bluewaterrider: "I'm surprised that a Skyfather like Zeus defeated Hulk when Zeus' Top-Tier son Hercules has lost to Hulk."
Perhaps but Superman's will stay healthier for a heck of a lot longer.
__________________ Darwin's theory of evolution is the great white elephant of contemporary thought. It is large, completely useless, and the object of superstitious awe.-Dr. David Berlinski, Philosophy
Most people believe Evolution not because they themselves are dumb, but cause they trust the "experts" who are feeding them evolutionary fast food, and so they don't bother questioning whether or not it's true.
Actually it was Jeph Loeb's Superman/Batman arc, roundabout issue 13, not Batman/Superman, a point I make only because people searching for this arc will suffer unnecessarily great difficulty using that as their term.
I didn't see it that way.
Batman did what he did with the aid of a Mother Box.
He also did it with Darkseid's tacit respect and approval.
And Darkseid still retained Xanatos-level control.
His promise only included making no direct intentional action against Kara.
It allowed for payback against Superman himself, however, and Darkseid sought such a short time later.