How can I find out the IO throughput of an external storage device connected to a MBP?
A nice leightweight monitoring tool is bwm-ng (installed with brew):

It shows network as well as disk activity:

$: bwm-ng -i ioservice
  bwm-ng v0.6.2 (probing every 0.500s), press 'h' for help
  input: ioservice disk IO type: rate
  /         iface                   Rx                   Tx                Total
  ==============================================================================
            disk0:        4063.49 KB/s            0.00 KB/s         4063.49 KB/s
            disk1:           0.00 KB/s         2579.36 KB/s         2579.36 KB/s
            disk2:        4063.49 KB/s            0.00 KB/s         4063.49 KB/s
            disk3:           0.00 KB/s         4000.00 KB/s         4000.00 KB/s
  ------------------------------------------------------------------------------
            total:        8143.14 KB/s         6584.49 KB/s        14727.63 KB/s


The example displayed is disk activity related to the command sudo asr ... (cloning an APFS disk0s2/disk2 to disk1s2/disk3).