Ethernet Shield: Maximum speed
It's not just the SPI what limits overall speed. You also need some proccessing power, and AVR based Arduinos lacks of it. It all depens what do you want to do with data.

If you need some serious processing power, it might be better to use something much more powerfull, like raspberry PI with real operating system. You can even run some web server at the speeds you can't even imagine with Arduino with the ethernet shield