Why USB harddrive is so slow under FreeBSD?

[Yuri]

I have FreeBSD-71 running with USB controller:
uhci0: <VIA 83C572 USB controller> port 0xa800-0xa81f irq 21 at device 
16.0 on pci0

I connected to it Toshiba USB hard drive HDDR500E03X:
umass0: <Toshiba External USB HDD, class 0/0, rev 2.00/1.03, addr 2> on 
uhub4
da0: <Toshiba External USB HDD 1.03> Fixed Direct Access SCSI-4 device

When I try to copy data with 'dd if=/dev/da0s1 of=/dev/null count=10000' 
it only achieves has 0.65MB/s transfer speed.

What's wrong, why is it so slow?

Yuri