I don't see any indication of problem with above. Do you?
Perhaps it would be useful if you do a comparsion on Windows machine, do a 'netstat -rn' on Windows too. And see if both machines use the same gateway. And do a ping to outside host, and compare the time taken for replies coming back. If they are the ping result and routes/gateway are the same for both machines, then next you'll have to check your port configuration on your Linksys router. It is quite possible your Linksys port is not communicating well with Linux box on layer two level, so far all we observed here on network level three, so it doens't tell you much problem on layer two. You might want to set the port speed to straight 10M or 100M both ends instead allowing them to do auto-handshake, and check full-dupex/half dupex thing too, try to avoid auto-detect speed. It is possbile one port/or your NIC is only capable of half duplexing while other port is trying full duplexing. Or one port is always trying to negociate port speed, and another port don't participate the same game, so there is a speed mis-match, as a result, you observe high latency and high packet loss on layer three. Try to some digging on layer two will be helpful. I heard stories that some manufactures had NICs bear the same MAC address, they thought a single customer would probably never end up having two NICs on the same network.
Good luck.

Hi M.I

The only problem I can see is the "DelayedACKs: 23 " it's too high for the amount of outgoings.
This suggests that something, i.e your switch, router or ISP's router is timeouts some of the packets sent buy the Linux box.

Make those changes I suggests earlier and see if it help.

/Raz

The changes that you've suggested didn't really make any noticable difference. I'll get to try using this machine on a different network in a few days, so I'll see if that improves my situation.

I'm now connecting to the internet via a DHCP based resnet instead of via a Linksys router (also DHCP) to a cable modem, and everything worked perfectly after plugging it in for the first time. The problems of extremely slow transfer speeds no longer occurs.