Software RAID – Rebuilding causes machine to lag

# cat /proc/mdstat

how to change resync speed so system is usable

# cat /proc/mdstat

md2 : active raid1 sda3[0] sdb3[1]
258855232 blocks [2/2] [UU]
[>………………..] resync = 3.0% (7814080/258855232) finish=4462.9min speed=936K/se