aha, it's good then, the module is OK. I think the adapter as well, it just probably is not meant to squeeze all the speed out of the module, it's for flashing only. finally, - writes are way slower than reads, plus going through additional translation (USB <-> eMMC) adds to overhead. also, maybe it runs in 1 bit (or 4 bits) bus width (as opposed to normal 8 bits), who knows.
ANT - my hobby OS for x86 and ARM.