Thanks for that, I didn't come across it in my searches.
When I disable bridging reads are 220MB/s+ which is more what I would expect. I do run VM's which was one of the features that brought me to Unraid in 2015, so disabling bridging is not an option. I did try with bridging enabled and no VM's powered on, same slow read speeds. The machine is also in the basement on the other side of the house, so running a second cable would be a pain.
*EDIT*
I created a new route for my local subnet to go out eth0, that fixed the issue. I am not seeing any knock-on effects after adding this route (br0 to local subnet seems to be permanent). Am I missing anything?
anibis2's post in Poor SSD Cache Read Speed 2.5Gbe was marked as the answer