I think you're on to something there with regards to the performance. Schneier was making some noise towards the end of the competition that none of the entries really improved on speed. ( https://www.schneier.com/blog/archives/2012/09/sha-3_will_be... ) Contrary to to article, hashing is used a lot, and there are a few areas where performance will be an issue for sometime (mobile space, and high-density virtualized environments) It may be that the key space was indeed deemed excessive, and of little added benefit compared to the rewards gained in greater speed.
As another mentioned further down, no one is simply going around busting up 256bit keys, and when it finally happens, it will be with a computational paradigm that will require a defense built to an entirely different set of requirements: out of the scope of this competition.
As another mentioned further down, no one is simply going around busting up 256bit keys, and when it finally happens, it will be with a computational paradigm that will require a defense built to an entirely different set of requirements: out of the scope of this competition.