Here is an easy way to either speed up or slow down the RAID re-synchronization speed rate on Linux. There is many reason why you would do that. Leaving your array rebuilding at full speed might dramatically slow down your server.
Set the minimum synchronization speed :
echo 102400 > /proc/sys/dev/raid/speed_limit_min
Set the maximum synchronization speed :
echo 102400 > /proc/sys/dev/raid/speed_limit_max
The value must be set in kilobytes. The value above will synchronize at 100 Mb/sec.