CDC NCM: question regarding dev->rx_urb_size

Hi,

I am facing a strange performance problem with a device. I am using the
"14-Mar-2012" driver found in linux 3.4.

The device announces a dwNtbInMaxSize of 32 kB, hence causing ctx->rx_max to be
32kB and as a result dev->rx_urb_size too, since:
dev->rx_urb_size = ctx->rx_max; // in cdc_ncm_bind()

The performance (transfer rate) are however miserable. If I however set manually
the dev->rx_urb_size to 16kB instead of 32kB, I get very good performance.

So my questions are:

1) what is this rx_urb_size setting for? Where is it used (in usbnet?). Could
you detail?

2) Why changing from 32kB to 16kB could cause such dramatic performance
improvement?

If you need additional info, let me know.

TIA, Loïc