SDperf.txt

                 SD/SPI performance
    512-byte block read/write, mbs = megabits/second
	2GB SanDisk on arduino SD/RTC shield or Sparkfun breakout and 8GB SANS

    SD/RTC shield on maple 72MHz
             maple spi           maple spi/dma
SPI clock   read    write      read      write
1.125MHz    0.75mbs 0.64mbs             1mbs   
2.25MHz     1.1mbs  0.91mbs             1.86mbs
4.5MHz      1.4mbs  1.2mbs     3.19mbs  2.0mbs
9MHz        1.6mbs  1.25mbs    5.1mbs   2.6mbs
18MHz       1.6mbs  1.3mbs     7.1mbs   3.06mbs

		    arduino UNO  avr328/16MHz   SD/RTC shield
                SPI         SD (with OPTIMIZE_HARDWARE_SPI)
SPI clock   read/write     read    write
2MHz          1.57mbs      1.4mbs  1.2mbs
4MHz          2.6 mbs      2.3mbs  1.7mbs
8MHz          3.8mbs       3.4mbs  2.1mbs

  teensy 3.0 at 96 mhz with Sparkfun SD and SANS 8GB card
SPI clk      SPI rd/wrt    SdFat SPI       SPI+DMA
2            1.5    1.0      1.8  1.1
4            2.7    1.4      3.2  1.5
8            3.9    1.7      5.3  1.9
12           4.5    1.8      6.8  2.2
16           5.0    1.9      8.0  2.1
24           5.5    2.0      9.5  2.3

   DUE 84 mhz  SD   (512byte blocks)  Sparkfun SD breakout
SPI clk      SPI rd/wrt    DMA rd  write
1 MHz        2.8    1.8      4.9   2.6     mbs
2            2.8             4.9
4            2.8    1.8      4.9   2.6
6            2.5             4.1
7            2.7             4.6
12           3.2    2.0      6.4   3.0
14           3.3             7.0
21           3.6             8.4   3.4
28           3.8    2.3      9.3   3.4

  errors at 42Mhz

See discussions of performance of 33mbs on DUE with large block reads.
   http://arduino.cc/forum/index.php/topic,135439.msg1018776.html#msg1018776
 and teensy 3 improvements:
  http://forum.arduino.cc/index.php/topic,128335.0.html