Just a quick tip. If you are using plesk check the /etc/nginx/nginx.conf file for this;
worker_processes 1;
The default value for this is auto and plesk overrides this to limit the number of worker processes.
I am beating myself up for missing this obvious cause, and am making my shame for missing it public.
Why, oh why did plesk think this was a smart idea?
My advice. Set this value to either auto OR if you have high DB/php load to a value half that of the number of cores in your CPU and adjust from there.