Thank you very much for the answer.
I am not sure about anything, all I know is that there are slowdowns, like when it calls functions that get enrolled users and their data and iterates over users, it takes about a second for each user! I don't know if it's due to calculations or input/output or something else.
It also had timeouts on quiz attempts.
So I'm looking into anything at all.
We also moved the sessions out of the db.
My big tables do have millions of rows, though.