Regarding the second comparison, it's easy to do trivially better than someone else's work if you use their work as a starting point. But that type of optimization doesn't really matter as much as the first one you mentioned: the ability to "take that which would have run on a CPU, and parallelize it to run on a GPU.". It has always felt quite natural for me to do that, so it was strange to hear that it's so hard for others. Why is it difficult for them, but easy for me?

The easiest way for me to solve this mystery is to try to port an example someone considers difficult. Do you know of any?