Generally web apps perform better because you load the app once and it gets cached by the browser. The ajax calls to get content reduces the data that you need

Generally web apps perform better because you load the app once and it gets cached by the browser. The ajax calls to get content reduces the data that you need to transfer. It also forces you to create APIs and reuse them and on the frontend side it provides much more flexibility to modularize and reuse components compared to the clunky widget interfaces available in server side frameworks. 404s are irrelevant to the fact that the site is using an web app or not. 404s exist for the longest time and they're as common in both cases.