Thank you for replying. After done several tries and searches, I find that this is a really common problem among fpm servers. I did seperate my data dir from the /var/www/html, which is presented to the public. And I even created a completely new instance to avoid any other factors. But the truth is the cpu usage comes back to 100% everytime. I tried different images (22-fpm-alpine, 21-fpm) but always end up in the same result.
Using Htop I can know that the main cpu comsumer is a command called 'php-fpm pool www ', every single of this command can comsume up to 50% of my cpu. I searched this term and found this is a persistent problem from very early on (a discussion here). I don’t get any other clue about how this happens and why this didn’t raise the attention of nextcloud team, since I think this is almost definitely going to occur when you were using php-fpm