Interesting speed benchmarks

On a lark, I just got a combo USB/Firewire external disk drive.  I ran
some crude benchmarks, and I was surprised by what I found.  This is
on a fairly stock -current kernel.

Firewire does around 40MB/s, while USB 2.0 maxes out at about 12MB/s.
This is with a simple dd command:

When the enclosure was attached as firewire, I got these numbers:

dd if=/dev/da0 of=/dev/null bs=1m count=1000
1048576000 bytes transferred in 25.867655 secs (40536183 bytes/sec)
1048576000 bytes transferred in 25.886887 secs (40506068 bytes/sec)
1048576000 bytes transferred in 25.880712 secs (40515733 bytes/sec)

when it was attached via usb (same disk):
dd if=/dev/da1 of=/dev/null bs=1m count=1000
1048576000 bytes transferred in 91.098764 secs (11510321 bytes/sec)
<gave up waiting for other runs>

and for comparison (apples to oranges, I know):

dd if=/dev/ad0 of=/dev/null bs=1m count=1000
1048576000 bytes transferred in 32.173160 secs (32591639 bytes/sec)
1048576000 bytes transferred in 32.139310 secs (32625965 bytes/sec)
1048576000 bytes transferred in 32.114549 secs (32651120 bytes/sec)

Summary:

firewire	40.5 MB/s
usb		11.5 MB/s
ata		32.6 MB/s

So why the huge difference?  This is all the more amaizing because
400Mb/s is 50MB/s.[*]

Warner

P.S.  This is on my amd64 laptop, which may be why ata didn't do so
well.

P.P.S:
ad0: 114473MB <FUJITSU MHV2120AT PL 008300A1> at ata0-master UDMA100
firewire:
da0: <Maxtor 6 B200R0 BAH4> Fixed Direct Access SCSI-0 device 
da0: 40.000MB/s transfers
da0: 194481MB (398297088 512 byte sectors: 255H 63S/T 24792C)
usb:
da1: <Initio 6B200R0 2.35> Fixed Direct Access SCSI-0 device 
da1: 40.000MB/s transfers
da1: 194481MB (398297088 512 byte sectors: 255H 63S/T 24792C)

P.P.P.S. M == 10^6 here.