I've bumped into the same problem... I expected at least 30MB/sec (240Mbps) from the device, as the physical hard-drives are a good spec (32MB cache, 7.2K RPM)
I've researched this nearly to death, and have found the apparent cause:
Many of the Buffalo LinkStation and TeraStation devices only have SATA-150 (150Mbps) connectivity to the physical drives. So even though the physical disks can perform at SATA-300 speeds (300Mbps), the controller on the Buffalo mainboard is only able to perform at 150Mbps (just under 20MB/sec)
If you take away some overhead for RAID throughput, and possibly some buffer throughput, let's assume the NAS performs at 14MB/sec (seems to be the average I've seen), that's just 112Mbps of actual performance. Not far off from the SATA-150 controller's best.
My only hope is to save up for a more expensive (unreasonably so!) SATA-300 or even SATA-600 capable NAS. I've seen throughputs of up to 100MB/sec, such as the Synology DS411+. But they're around £550-£600... so perhaps when that bonus comes through!
I hope this has been helpful in at least explaining the performance.