Server so slow

Start verifying your caches.

Then deactivate all plugins and see if the load decreases. If it doesn't, perhaps your site is under some kind of attack. Otherwise go activating each plugin again, and checking which of them causes your troubles.

If you find your problems to be not related to plugins you must find what's is causing the heavy requests. A good start point, perhaps:

cd /var/log/nginx
tail -f *.access.log | grep -w php

This will list all of your requests to .php resources in real time. hopefully this makes some bells ring.