USB is very heavy CPU load. There's an interrupt every 64 bytes, and the device itself is slow to read from and only 8-bit wide.

Ethernet is fast to read from, has one interrupt every 1.5kish (on a big ftp, for example), and is on a 16-bit bus.

Running usb & ethernet at the same time flat out may slow down ethernet a little and the extra usb throughput would be hard pressed to make up for it.

Hugo