Most of the older Ethernet shields have the wiznet w5100 chip. I have a wiznet WIZ820io with the newer w5200 chip. The w5200 has 32KB of buffers (vs 16KB on w5100), and will run at up to 33.3mbs (megabits/second) over SPI (0.3mbs on w5100). I added the support mods for the W5200 to w5100.cpp and w5100.h in hardware/arduino/sam/libraries/Ethernet/utility/. There is also a minor/optional modification to Ethernet.h. Those updated files and early performance results (wizperf.txt) can be found at
Simple UDP tests included 8-byte echo latency (microseconds), UDP 1000-byte packet sends from DUE, and lossless receive rate for 1000-byte UDP packets. The wiznet chip devotes 2048 bytes to each send and receive buffer (per socket). UDP is a lossy protocol. TCP will typically be slower, but it is reliable and adapts to available bandwidth.
Results include tests on the UNO and maple. On the maple and DUE, one can test SPI+DMA. The wiznet site only guarantees 33.3mbs SPI performance for the W5200. I got reliable results at SPI speeds of 28MHz, but errors at 42MHz. The DMA network tests for the DUE are still in progress and will be posted later on this thread at the git URL above.