If you have read How USB Ports Work, then you know that a USB port has a maximum data rate of 12Mbps. However, of that available bandwidth, an individual USB device can use only up to 6Mbps. And if you have several devices attached to the USB ports and they pump lots of data, this competition may further lower the data rate.
An Ethernet card, on the other hand, connects directly to the computer's bus. Ethernet cards come in two flavors: one that accepts 10Mbps, and another that accepts 100Mbps.