Comment by onion2k

every action including scrolling roundtrips to the server

As soon as you include a network roundtrip in anything you're opening up a Pandora's box of slow connections, slow DNS queries, network outages, what-if-the-user-is-on-a-train problems, what-if-their-IP-changes-mid-flow problems, etc.

Reducing the network calls in any app has upsides and downsides. It isn't really true that SPAs are faster to render (as your example proves) but rendering speed isn't the only thing that matters.