I notice when I stop the mysqld service, the cpu/ram usage drops very low. But when I start the mysqld again, it starts to rise and the sites are also not responding.
The limit of “Free” support end when I need to log in on a remote server that is not my own. You can send me PM if you want / need how ever if it won’t be “free”
What is the hardware config of your server… CPU cores, RAM, Storage, etc and how many sites are hosted?. This looks like typical out if memory problem
if it is wordpress, probaly wordfence is something to start. But this isnt related to hestia, so we cant provide any tutorials here. I would suggest to suspend the hacked web domain and restart the fpm service to stop active user processes.
systemctl stop exim or systemctl stop exim4 should do the job, but probaly your hacked wordpress is now spaming around. Still suggest to suspend the user or web domain, restart fpm or reboot the whole server to stop the hacked processes then probaly also clear the exim queue.