network speed will depend on the chip on the Ether shield and SPI speeds. The older w5100 ethernet module doesn't have a burst read/write mode so software has to do extra SPI transfers for each byte transferred. On the UNO, with SPI at 4MHz, I measured data rate of only 0.38mbs (megabits/second) for the W5100, whereas in burst mode (e.g. for w5200), I got 2.6mbs. Running the SPI at 8 MHz, I got .46/3.8 mbs (w5100/w5200).
For the leaflabs maple here are results for W5200, with different SPI speeds and using DMA with SPI
clock(MHz) SPI SPI+DMA
1.125 .7 mbs .9mbs
2.25 1.3mbs 1.8
4.5 1.6mbs 3.3
9 1.9mbs 5.5
18 1.9mbs 7.8
I haven't had a chance to perform DUE W5200 tests ....
UDP is capable of near wire speeds (for 10mbs and 100mbs ethers) from big-computer network controllers, but it can be an unreliable protocol over the Internet (packets can arrive out of order, duplicated, lost, or with errors).