Or if you want to measure the speed of the source itself:

  $ dd if=/dev/zero of=/dev/null bs=1M count=100
  104857600 bytes (105 MB) copied, 0.0237114 s, 4.4 GB/s

  $ dd if=/dev/urandom of=/dev/null bs=1M count=100
  104857600 bytes (105 MB) copied, 21.501 s, 4.9 MB/s

Also dammit Ubuntu with your Gibis.