This topic was on my mind for a long time.
Let's assume we have a typical web server, one in Node.js and the other in Java(or any other language with threads).
Why would node perform better (handle more IO/network based requests per second) than a java server just because it uses async/await? Isn't it just a syntatic sugar that utilizes the same threads java/c#/c++ use behind the scenes?
There is no reason to expect Node to be faster than a server written in Java. Why do you think it might be?
It seems the other answers here (so far) are explaining the benefits of asynchronous programming in JS compared to single-threaded synchronous operations -- that's obvious, but not the question.
The key point everyone agrees on is that certain operations are inherently slow (e.g.: waiting for network requests, waiting for disk/database access), and it's efficient to let the CPU do something else while such operations are in flight. Using several threads in your application is one well-established way to do that; but of course that's only possible in languages that give you threads. Many traditional server implementations (in Java, C, C++, ...) use one thread per request (or, equivalently, a thread pool to distribute incoming requests over). These threads can block waiting for, say, the database -- that's okay, the operating system will put the waiting thread to sleep and let the CPU work on another thread (handling another request) in the meantime. The end result is fairly similar to what you get with Node.
JavaScript, of course, doesn't make threads available to the programmer. But instead, it has this concept of scheduling requests with the JavaScript engine and providing a callback to be invoked upon completion of the request. That's how the overall system behaves similarly to a traditional threaded programming language: user code can say "do this stuff, then schedule a database access, and when the result is available, continue with this [callback] code here", and while waiting for the database request, the CPU gets to execute some other code. What you want to avoid is the CPU sitting around idly waiting while there is other work waiting for the CPU to have time for it, and both approaches (Java threads and JavaScript callbacks) accomplish that.
Finally, async/await (just like Promises) are indeed just syntactic sugar that make it easier to write callback-based code. Code using async/await isn't any faster than old-style code using callbacks directly, just prettier and less error-prone. It also isn't any faster than a (well-written) Java-based server.
Node.js is popular because it's convenient to use the same language for the client and server parts of an app. From a performance point of view, it's not better than traditional alternatives, it's just also not worse (or at least not much; in practice how efficiently you design your app matters more than whether you implement it in Java or JavaScript). Don't fall for the hype :-)