Here's a benchmark test using dual Compact Flash cards:
SpartaDOS X 4.43RC, dual 256MB, 80x Transcend Compact Flash cards, drive C: on disk 0, drive M: on disk 1.
Copying a 1,329K (1.29MB) file from partition C: to partition M: 49 seconds (26.58 KB/s).
Assuming SDX is using a ~30K copy buffer, that's about 44 burst reads/writes.