In the real world, all that matters is how the web app is actually built. There is always a bottleneck, and after you remove that bottleneck, there is always an

In the real world, all that matters is how the web app is actually built. There is always a bottleneck, and after you remove that bottleneck, there is always another. IO is just one of many things that can bottleneck an application, as your example points out.