Human brains are incredibly small, a few pounds of matter. Any bigger and your mother would be killed giving birth or you would take 10x as long to grow up. They are incredibly energy constrained, only using a few watts of power. Because any more and you would starve to death. They are incredibly slow and energy inefficient; communication in the brain is done with chemical signals that are orders of magnitude slower than electricity and use much more energy. And they are very uncompact - neurons are enormous and filled with tons of useless junk that isn't used for computation. Compared to our transistor technology which is approaching the limits of physics and built on an atom by atom scale.
That's just the hardware specs of the human computer. The software is hardly better. There are just more unknowns because we haven't finished reverse engineering it (but we are getting there, slowly.)
But beyond that, the human brain evolved to be good at surviving on the Savanahs of Africa. We didn't evolve to be good at mathematics, or science, or engineering. It's really remarkable that our brains are capable of such things at all! We have terrible weaknesses in these areas. For instance, a very limited working memory. We don't realize how bad we are, simply because we have nothing else to compare ourselves to.
Consider how even today, relatively primitive AIs are vastly superior to humans at games like chess. Human brains also didn't evolve to be good at chess after all. Even simple algorithms designed specifically for this game easily mop up humans. And play at a level of strategy far above what even the best human players can comprehend.
Imagine an AI brain that is optimized for the purpose of mathematics, or computer programming, science, or engineering. Or at doing AI research... Imagine how much better it could be at these tasks than humans. It could quickly solve problems that would take the greatest human minds generations. It could manage levels of complexity that would drive humans crazy.