poor gigabit ethernet performance with amd64 + sk0

I recently did a short file transfer over gigabit ethernet in both x86 and
amd64. On x86 it appears the limiting factor is the laptop I'm transferring to.
I get about 20MB/sec to the laptop in x86. But on amd64, I'm not sure what the
limiting factor is, because I max out at about 2.8MB/sec with the EXACT same
hardware. Here are the dmesg lines for that ethernet card:

skc0: <3Com 3C940 Gigabit Ethernet> port 0xb400-0xb4ff mem 0xfdc00000-0xfdc03fff
irq 17 at device 10.0 on pci0
skc0: 3Com Gigabit LOM (3C940)
sk0: <Marvell Semiconductor, Inc. Yukon> on skc0
skc0: [GIANT-LOCKED]

I'm positive that in both x86 and in amd64, the card is set up the same way,
with a 1500 byte MTU, and running at 1000baseTX full-duplex:

sk0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        inet 192.168.0.16 netmask 0xffffff00 broadcast 192.168.0.255
        inet6 fe80::20c:6eff:feba:a749%sk0 prefixlen 64 scopeid 0x1
        ether 00:0c:6e:ba:a7:49
        media: Ethernet autoselect (1000baseTX <full-duplex,flag0,flag1>)
        status: active

So I'm at a loss as to why it's so much faster in x86 than it is in amd64. Any
ideas?

Ken