@glyn.hudson - I am wondering if this is the same issue I have been struggling with.
I just started with a new SD card and a recently downloaded emonSD-03-May16 image. And I still have the “slowness”.
It seems like the slowness may be related to the apt-get update and apt-get dist-upgrade (or apt-get upgrade). So I am going to try the new everything again, but with no apt-get update and apt-get dist-upgrade (or apt-get upgrade). The only issue then is my Node-RED flows won’t work until I update the Node-RED software from 0.13.x to 0.14.x.