Are you talking about slow transfers to the external drives? If so, and if the external drives use USB 2.0, you might want to think about switching them to eSATA or USB 3.0 - that will make a big difference in transfer speeds.
If you're copying files from and to the same drive, that's probably a reasonable transfer rate. Remember that you're reading 20MB/sec from one file and writing 20MB/sec to the other for a total of 40MB/sec. On top of that the head will have to keep seeking back and forth between the two of them.