The units in echo are kB as in kilobyte.
Setting a high sync speed
echo 120000 >/proc/sys/dev/raid/speed_limit_min
This will increase the speed, note that sometimes a rebuild is slow due to current disk activity/iowait.
If that is not the cause then you may have a hardware issue (controller, cable or a bad drive).
Setting a lower sync speed
echo 1200 >/proc/sys/dev/raid/speed_limit_max