Yes, with stock tablet linux kernel maximum speed is approx. 7MB/s and with high-speed SD mode (48MHz) it is approx 12MB/s. So definitely 30MB/s is not possible. Also 30MB/s is not possible for SD even in theory. SD communication is in 4 bit mode and maximum defined bus frequency is 50MHz (SD specification). This gives 25MB/s maximum bandwidth of SD bus. Real maximum speeds are lower, see e.g. http://www.hjreggel.net/cardspeed/speed-cards.html#sd2 - Transcend 150x 4GB gives 21MB/s

On our tablet maximum SD bus speed is 48MHz (OMAP2 MMC clock 96MHz with integer divider) but CPU power and linux kernel and sd/mmc driver overhead reduces this to far lower speeds mentioned above. Also those are raw block device speeds, filesystem layer adds additional overhead too.