I did several tests using a random no-name brand USB stick I had around (swag from a trade-show).
I believe I see over 50Mbits/second throughput reading from the USB mass storage device.
Here is an example of a test I ran:
TEST1: Read 10MByte (80Mbit) file from USB memory stick.
# time cat /mnt/usb1/10mb > /dev/null real
0m 1.47s user
0m 0.01s sys
0m 1.08s(10MB * 8bits/B) / 1.47s =
54.4Mbits/second.
CPU load ?? = 100% * (1.08+0.1)/1.47 = 80%