Egad, man! Why would you use that other OS between such a sweet piece of hardware and the WWW?
It is not unusual to have transfer rates a factor of two or three lower going through that other OS. It is just not designed for networking. A factor of ten as you see probably means there is another misconfiguration problem, like the other box being set for 10 base T, perhaps. The EU has found that other OS discriminates against other browsers, too. If you fake out your browser to be IE whatever, is there an improvement? If only SMB transfers are affected, I would avoid the SMB transfers and use Linux on your gateway. You may have a bad cable or NIC. Look at ifconfig output to see errors. Use mii-tool to check speed settings.