Performance of USB over PPP

Hello all,

I am using usb-serial driver and got the PPP over USB running successfully using this driver. I am able to ping between the device and host and viceversa successfully. I am also able to do ftp. When i try to transfer a file of 1MB size from host to device, the transfer takes 1minute. For files sizes in the range of hundreds of KB, I get a throughput of 30kbps.
I wanted to know whether the performance of USB is affected by the PPP.

Thanks for your time..

Regards,
mohan.