Even in theory, producing json is just typically slower than producing HTML.

It seems unintuitive, but traversing an object tree using reflection to generate json is just slower than using an HTML template, which is probably a rope data structure.

Rendering the template or JSON response is the last and probably often the least computationally expensive step though.

I guess the rendering itself is pretty much the same for both SPAs and MPAs, since in the end it can be pretty much the same markup, but the SPA is also at a disadvantage here because it has to combine template/components + JSON to generate the HTML :/

That reminds me of when I first benchmarked a Rails app and noticed that JSON rendering was significantly slower than the remote database call. No lie, jbuilder was taking about 80% of the request time for about 90% of all endpoints.