I have a DS410j with DSM 4.0-2198 and I also get this problem.
I tried to do a copy between 2 external USB hard disk (ext4 to ext4, tested before ext4 to ntfs and slower; both western digital - one disk + textorm case & one My Book essential usb3) and get very slow results.
'hdparm -tT' returns 200-230/25-28 MB/sec but rsync copy gets 3-5MB/sec with top at 50% usr, 45% sys, 1-4% i/o.
on a another box with lubuntu, 'hdparm -Tt' return about 270/30 MB/sec results but copy still stay between 2-4 MB/sec ...
> cat /sys/block/sd*/device/max_sectors
from what I saw on the web, this could be increased to 1024 but for me, it has little improvement, maybe 1MB more.
'mount -o remount,async' doesn't seem to make any changes too.
That makes a one TB copy to about ~58 hours ... while I got some network transfer on gigabit ethernet at ~20MB/sec ...
Any ideas how to improved this ?