nano /etc/sysctl.conf
#################NOTE ################ ##You are limited by CPU and memory too
# ###########################################
dev.raid.speed_limit_min = 50000
## good for 4-5 disks based array ##
dev.raid.speed_limit_max = 2000000
## good for large 6-12 disks based array ###
dev.raid.speed_limit_max = 5000000
https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html