I think the main issue here is that we don't do buffering / pipelining
for bulk transfers at the moment. We grab a single transfer request
from the guest, pass it to the kernel, when it is done pass it back to
the guest, then look look for the next one. Instead we could queue up
all transfer requests from the guest to the kernel, which would give a
noticable better throughput. The qemu usb subsystem can't handle that
(yet). Fixing that is one the TODO list though.
Additionally all bulk xfer processing is done in the 1000 Hz frame
timer, which combined with the above limits the number of packets to
1000 packets per second (and direction), i.e. with a MTU of 1500 you'll
get 1500 * 1000 = 1.5 MB/s max. This you can expect to get with the
current code.
Add some protocol and other overhead to the 7 MBit/s you are actually
seeing and the numbers are pretty close, so there isn't much room to
improve things. The only option I see is to operate the device with an
larger MTU if the usb device and your network setup can handle that.