To see your Linux kernel speed limits imposed on the RAID reconstruction use:

cat /proc/sys/dev/raid/speed_limit_max
200000
cat /proc/sys/dev/raid/speed_limit_min
1000

In the system logs you can see something similar to:
md: minimum _guaranteed_ reconstruction speed: 1000 KB/sec/disc.
md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for reconstruction.

This means that the minimum guaranteed speed of the rebuild of the array is approx 1MB/s. The actual speed will be higher and will depend on the system load and what other processes are running at that time.
In case you want to increase this minimum speed you need to enter a higher value in speed_limit_min. For example to set this to approx 50 megabytes per second as minimum use:

echo 50000 >/proc/sys/dev/raid/speed_limit_min

The results are instant… you can return to the watch window to see it running, and hope that this will finish a little faster (this will really depend on the system you are running, the HDDs, controllers, etc.):

watch cat /proc/mdstat

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Bitmap Option

Bitmaps optimize rebuild time after a crash, or after removing and re-adding a device. Turn it on by typing the following command:
# mdadm --grow --bitmap=internal /dev/md0

Once array rebuild or fully synced, disable bitmaps:
# mdadm --grow --bitmap=none /dev/md0

Source: http://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html