I'm trying to think of a good analogy because I think I am being misconstrued.
The first point I am trying to make is that Both computers and Brains are physical objects, each of them are powered by electricity and make calculations based on the flow of that electricity through their components. At a fundamental level it is impossible that a synthetic brain can not be created as their is no magical, or impossible element a brain possesses that can not be simulated.
The next point Is that pure computational power does not "cause" intelligence, it is the application of that computational power from which intelligence emerges. A Lamborghini has more Horse power than a tractor trailer, but the tractor trailer applies it's horse power to be able to pull much heavier loads (through torque) than the lamborghini does. While this statement can indicate that a brain actually processes data slower, it is my opinion that a human brain has more raw processing power than current computers.
Comparing the speed at which an action is done between two machines is not indicative of which machine is working faster. For example, if you wanted to retrieve data from a database you have a multitude of options. Suppose one machine does a simple "Select X,Y,Z from TBL" while another uses a complex and well designed ORM. The first, naive implementation will execute more quickly simply due to the reduced number of instructions that need to be processed, but the ORM version will be much more flexible and complete.
The "intelligence" algorithm, will be/is orders of magnitude more complex than any algorithm a computer has ever been given, which is why a computer can complete particular problems more quickly than a human... but the "intelligence" algorithm is still nothing more than an algorithm, which when properly understood will be just as easily followed by a synthetic brain (computer) as a human brain.
The comparison of computational power between the human brain and a computer was meant to illustrate the following:
If the computer is running the simple select, and the brain is running the ORM and the brain is outputting the solutions more slowly then the computer... then I believe that the human brain running the simple select would easily outpace the computer...
This doesn't rely on a definition of "what is intelligence"... no matter how it is defined, it will ultimately be an algorithm, which a computer will be able to run.
You say that a computer can only do what it is told to do, and while I completely agree, I also believe that there exists a method in which we could tell a computer to learn, comprehend, imagine, innovate and create. And I believe this because i think that at the most fundamental way our brains must work as some form of computational device that is running a series of definable algorithms.