Slackware 11.0, 2.4.33.3 kernel. I have an ibm pro 100 mb nic on the motherboard (Toshiba 1415 s173 laptop), and a Belkin wireless nic with an Atheros chipset. Both work. I did some performance tests using iperf today, and found that the onboard nic ran at 96.5mbps. I plugged in the wireless nic, which is currently connected to an old Netgear B router, and it ran it 5mbps. However, after plugging in the wireless nic and running the test on the ibm 100mb nic, it also ran at 5mbps. This only happens when the wireless nic is plugged in. And if I unplug the wireless nic, it goes back to 100mbps. Why? Does Linux route traffic from/to the 100mb nic through the wireless nic when it's plugged in?

Sounds like you messed up the routing. If you do an iwconfig ath0 down (or sth like that) does the same happen?
What I mean is: Is it just the presence of the wireless nic that mekes that difference or is it the routing between the two nics. Also remember that atheros runs very well at 802.11a and 802.11g but really HATES 802.11b.

PS: A guy had a similar problem and solved it by disabling reverse path filtering.