There's quite some space between "100% no JS" and "full SPA"; many applications are mostly backend template-driven, but use JS async loads for some things where it makes sense. The vote buttons on Hacker News is a good example.

I agree a lot of full SPAs are poorly done, but some are do work well. FastMail is an example of a SPA done well.

The reason many SPAs are slower is just latency; traditional template-driven is:

- You send request

- Backend takes time to process that

- Sends all the data in one go.

- Browser renders it.

But full SPA is:

- You send request

- You get a stub template which loads JS.

- You load the JS.

- You parse the JS.

- You send some number of requests to get some JSON data, this can be anything from 1 to 10 depending on how it was written. Sometimes it's even serial (e.g. request 1 needs to complete, then uses some part of that to send request 2, and then that needs to finish).

- Your JS parses that and converts it to HTML.

- It injects that in your DOM.

- Browser renders it.

There are ways to make that faster, but many don't.