"And HTML template rendering on the server can be faster, because for client-side rendering you need at least two roundtrips to the server (fetch template, one; fetch content via JS, two), and for complex apps this becomes tens of HTTP requests (side panels, user context, footers, menus etc.), versus just one roundtrip for a server-based approach."
Nope.
The template fetch is only the first time.
Because the template/HTML is either cached on the browser or is compiled/inlined with the Javascript with the build pipelines.
"Request/response lag is critical in the way an app is perceived."
Yes it is and it is worse with server rendered pages.
A typical size of a page fully dynamically generated on the server side could be a few times larger than that fetching of data/content.
Nope.
The template fetch is only the first time.
Because the template/HTML is either cached on the browser or is compiled/inlined with the Javascript with the build pipelines.
"Request/response lag is critical in the way an app is perceived."
Yes it is and it is worse with server rendered pages. A typical size of a page fully dynamically generated on the server side could be a few times larger than that fetching of data/content.