I wonder where this myth that "[websites] on the internet [are] I/O bound, not CPU bound" seemly implying that optimizing CPU usage is a waste of time, comes from.<p>Since the I/O bound generally comes from latency, rather than total throughput, the number of concurrent connections a single webserver can handle is often proportional to how much memory and CPU resources each connection uses. If you have an asynchronous design for your server, concurrent connections don't cost physical threads - they just cost the bookkeeping overhead in the kernel. The faster you can switch between those connections, and get finished with work on them when I/Os complete, the fewer physical machines you need to serve a website to a given number of users.<p>Or to put it another way, the saturation point of processing asynchronous I/Os is CPU-bound, even when speeding up individual requests is I/O-bound.