It's quite likely that even non-superpowered Kryptonians are significantly smarter than humans. I seem to recall Kryptonian kids studying advanced concepts at relatiively early ages.
Perhaps Kryptonian brains are more complex than humans, with far more neuron interconnections between areas of the brain. The major limiting factors would be the speed of electrical impulses and perhaps heat buildup from intense brain activity -- electric impulses have to generate some waste heat, which is then trapped inside the skull. Bloodflow would assist somewhat in heat dissipation, but one must also assume that hyper brain activity requires more oxygen to nourish the overworked brain cells. So the circulatory system affecting the brain would be yet another limiting factor on Kryptonian super-intelligence.
But what happens under yellow sun exposure? Now-invulnerable Kryptonian brain cells can handle activity levels that would literally "burn out" human gray matter or result in massive strokes. Neurons might even become superconductors, allowing brain impulses to travel without any electrical resistance. At this stage, the only significant limitation is the speed of light.
However, we've seen Superman perform instant calculations that in theory couldn't be accomplished in time. Even massively interconnected superconducting neurons couldn't fire fast enough. So how can Superman do this? Is it possible that his brain operates FASTER THAN LIGHT? Or is there a previously-unidentified psionic ability such as precognition that allows Superman to begin figuring out a solution in the back of his consciousness EVEN BEFORE THE PROBLEM PRESENTS ITSELF?