Playing more with hud I can see that there is a 1 to 1 correlation between the
pauses and spikes in num-bytes-moved. The scale on the graphs did get squashed
a bit by outliers, which seemed a bit random sometimes - I saw 330 MB on one
run - but anyway here's a couple of screens  - I got these using sleep 120 &&
xwd -root ... the first one landed on a scene change so is black.