I changed out the cable and there was moderate improvement but still slow compared to the results others are getting. To be fair, the Bananas is connected by Cat5e to the router, but all other clients talking to it are on wifi. The wifi itself is an Engenius External Wireless Access point that's giving me trouble. Internet speed tests are in the 50 Mb/s range, but it has a bad habit of kicking my laptop off the network when I'm moving larger files onto the NAS, and the only way to fix it is to reboot the WAP. I'm thinking there must be something in the setup/config of the WAP that's causing it to bottleneck the NAS.
If anything, this is a great learning experience.