There shouldn't really be any significant filesystem code involved, since once `dd` has opened the files, it should have handlers for those devices more-or-less

There shouldn't really be any significant filesystem code involved, since once `dd` has opened the files, it should have handlers for those devices more-or-less directly in its descriptor table. Once you have a descriptor to a pipe or device, there shouldn't be any filesystem-level checking in the middle of your reads/writes; all you're doing is filling/emptying buffers. And given that I can write a program that makes 132 million calls per second to the glibc `putchar` function (which also buffers), I'm pretty sure there's a lot of time that can be shaved off as we start to replace the system call mechanism with plain function calls.