When you start talking about multiple concurrent connections, and considering performance of an application as a whole, Async I/O at the application level offer

When you start talking about multiple concurrent connections, and considering performance of an application as a whole, Async I/O at the application level offers basically no speed benefits over any kind of Sync I/O runtime that can run in parallel. What an Async I/O model can do is allow the amount of resources consumed by parallel execution contexts to be reduced - therefore allowing you to service more concurrent requests in parallel. But not necessarily any faster, if I/O is your bottleneck in the first place. It can also be used to give more predictable performance under loads with i.e. response times, if the responses are not dependent on I/O operations to complete.