Log download speed is too slow. How to speed up?

My ArduPlane installed on a Durandal FCU shows very slow log data downloading speed. It is around 20KB/sec. I download the data through USB and QGroundControl.

When I use same ArduPlane with a Micro Pix FCU, the down loading speed is around 250KB/sec with same USB and QGroundControl.

I can not understand the reason why there is such big difference.
Could anyone teach me why and how I can speed up the Durandal’s downloading speed?