One of the first limits you'll find with running NGINX at scale is the defaults for the worker processes and connections. At a low level, an NGINX worker process is the dedicated event handler for processing all requests.
The defaults for most NGINX installations are 512 worker connections and 1 worker process. While these defaults work in most scenarios, a very busy server can benefit from adjusting these levels to suit your environment. There is no one-size-fits-all scenario when it comes to the correct values, so it's important to know where you're hitting limits and therefore, how to adjust to overcome them.
Setting the limits too high can result in increased memory and CPU overhead, which would have the overall effect of reduced performance rather than increasing it. Thankfully, NGINX will log when it hits certain limits...