iTunes download on NAS WD MyBook Live - super slow!

I have a WD MyBook Live with latest firmware - 2TB. Ever since I moved my iTunes media folder to the NAS performance is great, except my download speeds are crazy slow! I have a 450MBPS 5ghz WIFI connection. When transferring to and from my Macbook Pro to the NAS I get speeds in the region of 30MB/s. Yet when I am downloading with iTunes I get 1MB/s!

If I download straight to my HDD ( tested by making another iTunes folder ) I get super fast speeds that pretty much max out my 120MBPS Virgin Media connection. So what gives? I tried both making an alias and also a symbolic link point downloads to a different location, but iTunes does not recognise this.

Any suggestions?