Make sure you're using oflag=direct for writes and iflag=direct for
reads to avoid Linux buffering when using dd to write/read SD cards for
speed testing.
Once you introduce a USB reader into the mix, you're now at the mercy
of that USB reader's abilities. Using the MMC hardware on a SoC will
probably give you a better chance at high performance, although not
always.
Can you run flashbench [1] on this card? There's a lot more to SD
cards than raw bulk reads and writes (which is how they're meant to be
used) since rarely do raw bulk reads and writes actually happen if
using an SD card as a boot disk.
[1]:http://git.linaro.org/gitweb?p=people/arnd/flashbench.git;a=summary
Send results here and to the flashbench-results list, please, if you do
run it. See the archives for that list to get an idea of how to run
it, if it's not clear.
Thanks,
Andrew