Try setting TCP delay_ack to 0. Default is 3, which causes handshake problems with the ReadyNAS, causing severe performance penalty. You can set the delay_ack with the following command from the OS X terminal:
sudo sysctl -w net.inet.tcp.delayed_ack=0
If you want the setting to stick when you reboot, add the line to /etc/sysctl.conf. For reference, see: http://julipedia.blogspot.com/2006/02/samba-performance-under-mac-os-x.html http://mail-index.netbsd.org/tech-net/2003/01/28/0004.html