I recently purchased a Synology 1815+ NAS to increase my home network storage a bit and the performance was not what I was expecting out of the box. There were a few things that I identified to try and get things working a little bit faster. I was able to achieve roughly 50-75MB/s during volume growth and reshaping.
Step 1: Increase Stripe Cache Size
This will increase the lower min speed to consume for the raid volume reshaping/rebuilding. You will still hit a limit based on your spindle speed and CPU throughput.
echo 32768 > /sys/block/md2/md/stripe_cache_size
Step 2: Increase Min/Max Throttler for the RAID
echo 100000 > /proc/sys/dev/raid/speed_limit_min echo 100000 > /proc/sys/dev/raid/speed_limit_max
Step 3: Validate Current Performance (Resync)