Tim
Excuse me my side entry. ;) The OP seem to be somebody with a history of OSS, Linux and community engagement. (@Alejandro, if you are who I think who you are, add few things from your CV to the profile here.) What I want to say is that he is aware of the general principles. And he was given Performance and Performance FAQ in the same discussion.
Since he is from a developing country he has limits on lavishly spending on server resources, I believe. He has invested on a 12-core CPU and 24 GB VM VPS. That is something. (I searched for similar VM in another developing country and found they are as expensive as in Switzerland.)
What I understand from this new formulation of his enquiry is that he wants to get more out of that VM by investing on an additional storage volume and wants an answer to the question, "Which is better, move the database to the new volume or the moodledata?" From what Ken and Brett wrote, I think the answer is to "put the database on the faster volume". @Kenn, Brett, correct me if I'm wrong.
About the load tests, were you too modest to link to your early work? ;) For the newcomers, I mean https://tjhunt.blogspot.com/2013/05/performance-testing-moodle.html, which is linked from Performance_recommendations.