I'm glad it runs fast, and I hope that all OS utilities are optimised (and tested, of course!) instead of making their source code pretty. The fact is, most people want to use programs, not read them.

I actually checked just now, and it looks like you'd be making twice as many system calls with /dev/zero compared to generating the data locally:

    strace dd count=1 bs=512 if=/dev/zero of=test
    open("/dev/zero", O_RDONLY)             = 3
    dup2(3, 0)                              = 0
    close(3)                                = 0
    lseek(0, 0, SEEK_CUR)                   = 0
    open("test2", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
    dup2(3, 1)                              = 1
    close(3)                                = 0
    read(0,"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
    write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
    close(0)                                = 0
    close(1)    

The difference matters if you're benchmarking e.g. some new SSD compared to a tmpfs on a machine with 100+ GB of RAM. It's always better if the tools have less overhead, because the comparison is more meaningful.

Also consider that it can be faster to write to a local network than to disk. I've never done it, but I imagine that the kernel's not going to want to deal with your /dev/zero calls if it's spending all of its time writing to a 10GB switch. I can imagine some very specialized storage servers that could spend most of their time writing from memory buffers to a network switch, or if you're troubleshooting a slowdown in the networking itself.

    yes(){ while :; do echo "${1:-y}"; done; }

    import sys 
    while True:
      if len(sys.argv) > 1:
        print(sys.argv[1])
      else:
        print("y")

  while:; do echo "y"; done

Just because you say so?