How to reduce (Tweak) Synology Rebuild time - Use at your own risk!

echo 90 > /proc/sys/vm/dirty_ratio
echo 80 > /proc/sys/vm/dirty_background_ratio
echo 6000 > /proc/sys/vm/dirty_expire_centisecs
echo 4000 > /proc/sys/vm/dirty_writeback_centisecs

For the next two, change "sda" to the device name; you may need to repeat for each device.
echo deadline > /sys/block/sda/queue/scheduler
echo 16384 > /sys/block/sda/queue/nr_requests

For the next two, change "md2" to the volume name if different; you may need to repeat for each volume.
echo 524288 > /sys/block/md2/md/stripe_cache_size
blockdev --setra 4096 /dev/md2

The following line can only be enabled while the array is healthy
mdadm --grow /dev/md2 --bitmap=internal

Edit /etc/sysctl.conf to add this line
dev.raid.speed_limit_min=100000