Questions about using usbnet.c

Hi all!
When I use dev_queue_xmit( ) to send packets directly to the usb device from a netfilter hook( in a ipt target function),
( I want to forward packet from eth0 directly into usb0, so I do it like that)
the rate is so slow that dev_queue_xmit always returns 1 which means droped ( 200kbits/s at most).
But when I used a user-space (TCP) program to send packet, and receive on the other machine,
the rate grew up to 6Mbits/s.
Is there any limit on using dev_queue_xmit( ) directly ? Regards,
yangrunhua