It's not a network issue, I used LAN speed test for my initial investigations. I have no cache drive so no problems there. As a work around for my particular issue, I moved from using Plex on docker, which was giving me read speeds of 8Mbps, resulting in choppy playback, to putting it on a VM, which as I said above has no such performance issues, and now I am getting 40+Mbps and no more chop. Note that this is the exact same physical machine, physical network card and physical network. As I said before, Windows and Linux VMs perform brilliantly, it's only trying to access the share from the unraid itself or from inside a docker that's got problems.
It's a fairly fundamental problem for anyone with this issue, I could get around it by mounting all my shares through a VM but I don't want to have to do that, so I hope it's being looked into