We're getting about 256Kbit/s absolute maximum through these boxes, whereas on the Ciscos we can sustain 8-10Mbit/s no problem in some tests, and burst much higher.
We've had multiple Shorewall setups and could get 8-10Mbit/s (link was
10Mbits) on a K6-500 with ~1000 rules.
We get a 100Mbits on a Sempron 2400 with ~1000 rules.
(Dual 100Mbit ethernet)
As your cpu usage is rather low, you could be getting packet loss.
We had an issue in the past with a upstream ethernet interface set at
10Mbit/Full duplex, but as the firewall could not negotiate, it defaulted to
10Mbit/Half duplex. We got about 5% packet loss in that scenario.
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Jean-Francois Pirus