Hey,
Are all the machines connected via a gigabit switch? Some switches are labeled as 10/100 and others 10/100/1000.
Another thing is the specs on the NAS. I know my NAS has some terrible bottlenecks within it (Western Digital ShareSpace). I forget what exactly it is but there are lots of complaints that I dug up when trying to troubleshoot it myself. My NAS has similar issues; lag in the material editor when pulling a large HDRI from the NAS, also lag when initially showing the folders in the network drive. I plan in the future to get a Synology NAS drive which I’ve heard great things about.
Have you tried just setting up a shared partition on a node to test the speed directly between a node & workstation? I have one node that shares an extra HD with my workstation and transfering data from that location is quite snappy.
Lastly, Teracopy is a great app for windows to see the transfer speed when doing these types of tests.
Hopefully someone else has some ideas to chime in on