Whats probably at issue here is the CPU utilization. Heres the relevant chart of CPU utilization.
The USB drive posts a whopping 66.6% CPU utilization -- higher is definitely not better here, folks.
Note that this CPU utilization is measured during a situation where the drive is blasting the maximum amount of data across the bus (high-speed sequential reads). In situations where you might be reading and writing from the drive in more random patterns, you probably wont notice such a severe CPU hit.
If you think about it, 480 megabits/sec -- the theoretical maximum for USB 2.0 -- is around 60 megabytes per second. Even if you postulate a 20% overhead for encoding, then you still should see around 48MB/sec, assuming no other activity on the USB port. However, the lack of DMA necessarily means the CPU gets heavily involved in data transfers -- which will translate to a lower maximum transfer rate.