I'm also puzzled as to why the benchmark of the monstrosity would have been faster. How is it not doing the exact same work as read()'ing 4KB, and then writing

I'm also puzzled as to why the benchmark of the monstrosity would have been faster. How is it not doing the exact same work as read()'ing 4KB, and then writing out the same to the destination file? And, as you said, 3x the system call overhead doesn't seem good, either....