In the old days to disable periodic RAID checks, which can degrade performance, you would get rid of /etc/cron.d/mdadm . With systemd creep, these days you need to

for svc in mdcheck_start.timer mdcheck_continue.timer; do systemctl stop ${svc}; systemctl disable ${svc}; done

This works on Ubuntu 20.04. Probably also on other systemd managed systems.