It may be, other possibilities include a separate socket connection for data (like ftp). USB has other limits like interrupt latency, as it interrupts for every 32 bytes of data (mk1) or 64 bytes (mk2). The protocol gets nearer maximum possible throughput on USB than on ethernet - max USB throughput is limited by the speed we can access the USB slave controller device (which isn't fast - the chip's design won't allow it :( ).

Hugo