Memory bandwidth is not the main concern here since they explicitly mention that executing more CPU instructions kill performance far quicker - in this case. As

Memory bandwidth is not the main concern here since they explicitly mention that executing more CPU instructions kill performance far quicker - in this case. As for mmap, perhaps you would like to present your own benchmark that contradicts the article. But anyway, I highly doubt that either of the basic things you mentioned are news to the implementors. The implementation is supposed to run on all kinds of processors with varying architectures so making the implementation portable is bound to leave performance on the table. Not to mention other constraints like maintainability and meeting the performance criteria for the common use case/workload.