To my knowledge there is not existing code that does benchmark the speed of the Arduino Ethernet Shield. Theoretically you could just use the webserver example of the Ethernet shield and use web server benchmarking programs but you'll probably would get other results if you would do the same with direct socket programming.
If you wanna do the above google for "web server benchmark". You'll get lots of results, you have to decide which one is best for your case (there is no benchmark that can do that "in general").