[matt4077]

There are some impressive numbers (from google I think) about conversion/retention/bounce rates in relation to page load times. I believe it was about a 50% reduction for every extra second.

Page loads are mostly download (bandwidth-limited) and layout. For many actions, you can reduce drastically reduce data transfers if only the actual data that changes is transferred (no markup/images/content already loaded). Similarly, only a small part of the layout may change. That can result in anything from doesn't really matter (newspaper) to "this project wouldn't make any sense without it" (google docs).

[virtualwhys]

Given that SPAs tend to have payloads in the MBs, high conversion rates must be based on the end user having a very fast broadband connection. They probably wouldn't stick around for the initial page load otherwise.

Modern browsers are very good at caching resources. For server-side apps the browser is often just downloading the html for the page and nothing else. Compare that with downloading json from the server and applying changes to the virtual DOM. The difference is neglible wrt to speed and is, IMO, oversold.