Mautic How to increase performance

My Mautic version is: 4.4.4
My PHP version is: 7.4
My Database type and version is: MariaDB MySQL
Mail client: Roundcube
Server managed with Virtualmin and Webmin

My problem is:
Once we reach large scale of emails (both sending and receiving replies),
Server becomes extremely slow, Mautic and roundcube become slower and a lot of things tend to break down in mautic which need manual fixing (but this is not concern of this thread for now)

We would like to know how can Mautics performance be improved (how fast it is loading, how fast emails are being sent, etc).

Even if you are not using virtualmin to manage your server, we would like to hear your general advice which helped you on your server.

For example we are considering allocating more RAM to MariaDB in hopes of speed up in both Mautic and Roundcube.

Any other suggestions would be appreciated.