Yes, you can add more processes (or threads), but more processes (or threads) blocked by IO equals more context switches which equals drop in performance. Not a

Yes, you can add more processes (or threads), but more processes (or threads) blocked by IO equals more context switches which equals drop in performance. Not a big deal if your app is distributed and you scale horizontally, but most apps don't need scaling that way and async fits the bill without adding unneeded resources. If your workload is IO bound (and most of web development is) async improves performance in any case.