That's where the nonblocking comes in. It relates directly to scalability. It doesn't improve performance, but it allows more requests to come through while you are waiting for external resources like databases to return results (by far the most common cause of slowdowns on web servers).

Performance and scalability are not always a tradeoff. When you increase performance at the runtime level, you don't decrease scalability. However it allows you to serve more requests in the same amount of time.