I have been using an old 1ghz 1 or 2Mb PC to run netbsd 5 and act as a firewall for a number of years. It is not used for anything but this and Probably your CPU is adequate, and either there's something wrong unexpected, or those interfaces are too crufty for 100 Mbps. If you have PCI slots and can try some other kinds, especially if you can find an intel card that is fxp(4), or DEC that is tlp(4), it might help. Run 'netstat -s' on the gw before and after the speed test, and diff the results. Try running wget or something on the gateway itself to download a large file. It's remarkable that you are getting 80-90 Mbps on a cable modem.
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