Hint: always use a non-default block size (unless you have a reason not to)

   $ dd if=/dev/zero of=/dev/null bs=1M
   ^C55034+1 records in
   55034+0 records out
   57707331584 bytes (58 GB) copied, 6.04479 s, 9.5 GB/s

And this is a 7 year old laptop. I could get blistering fast speeds writing to /dev/null on my latest 6 core i7 ;-)