Sometimes I think human intelligence might be perhaps the theoretical limit on how intelligence an organism can be. For one, think about how limitless our imagination is. Now someone might object and say, "No way is human intelligence the limit! Look how fast computers can calculate and !"
Well, computers at the moment aren't performing any mental feats we couldn't do. They're just performing them much, much faster. Second of all, sorting data in a brute force way isn't intelligent behavior. Thirdly, I think perhaps our consciousness adds a "cognitive weight" if you will that couldn't co-exist with the kind superspeed calculating feats a computer can do. Adding consciousness to our "intelligence" is very much like adding mass to an object in motion. Think of a computer as the speed of light and the human brain as a jet. The speed of light is so fast because it's unen bered by any kind of mass, existing purely as a wave. More mass, slower speeds. But the trade off is, well, more mass (consciousness).
To clarify, much how like an object with a large mass can't travel anywhere near the speed of light, an intelligence with consciousness can't "calculate" above a certain speed.