The database loads the CPU at 100%. What can be wrong?

Question

MariaDB

Salute to everyone, I have been suffering with the database (MariaDb) for 3 days now. I must say right away that I am poorly versed in this issue, but I honestly tried to solve it myself. In general, there is an active project, stands on Intel Xeon E5 (cores 2 vCPU).
Server version: 5.5.56-MariaDB.
At first, everything was fine, but now the project has grown and the user base has grown.
At the moment, the situation is such that if 30 users, for example, try to activate a promotional code, then the CPU flies up to 100%, the site simply does not open.
I have already optimized queries as far as I can, re-read about indexes and so on, optimized heavy tables with the help of Optimize and the same picture still happens.
Another important point, if you go into PMA and try to roam there, then periodically (very often), you can catch an endless load, the page will not load until you refresh it. This happens when entering the table, or when exiting the PMA main screen, or when entering the search, it doesn’t matter, it just throws it into a dead load and this, it seems to me, is the main problem, but its source is unknown to me.
The machine also has proxying (DDoS protection), but it does not naturally affect local requests. Thank you all in advance!

Answer 1

At first, everything was fine, but now the project has grown and the user base has grown.

Was it planned at the design stage?
Server settings are default? Did load testing at the deployment stage show anything like this?