very slow NIC

[TOPCAT CONSULTING]

I have an ethernet card that just doesn't seem to be performing well in
both directions (in and out)....when I access this NIC from another
machine on the LAN, file transfer is extremely fast, but when I use this
machine to access another machine on the LAN, I can barely get 10 kb/sec
out of it!

Anyway dmesg.boot reports the card as:

rl0: <RealTek 8139 10/100BaseTX> rev 0x10 int a irq 9 on pci0.13.0
rl0: Ethernet address: 00:20:18:88:96:47
rl0: autoneg complete, link status good (half-duplex, 10Mbps)

So, is full-duplexing not turned on or something?  How do I turn
full-duplexing on?  Or maybe that's not the problem?