Well, that was a quick test to check the obvious. There is definitely something wrong with your server - a 4 vCPU, 16 GB RAM machine must easily manage 30 concurrent users.
Now that eliminated, you have to systematically go through all the https://docs.moodle.org/33/en/Performance_recommendations. Also post the https://moodle.org/plugins/report_benchmark here.
Hardware and performance
Server Slowness is causing us badly
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