Telnet to the Synology NAS, login in a root. (You may need to enable Telnet Server first)
Raid Stats Command
cat /proc/mdstat
Default Raid Values
cat /proc/sys/dev/raid/speed_limit_max
200000
cat /proc/sys/dev/raid/speed_limit_min
1000
Improve Rebuild Speed – increase the speed_limit_min value.
echo 40000 > /proc/sys/dev/raid/speed_limit_min
check speed with
cat /proc/mdstat
Max speed on my DS1812+ with DSM5.1 was just under 40000K/sec
So 3TB Disk took approx 20hrs to rebuild