I wrote an essay about this here:

https://htmx.org/essays/splitting-your-apis/

Also, producing JSON often involves reflection and can be more CPU intensive than straight templating (w/ fixed properly lookups) depending on your technology. Finally, the size delta between JSON and hypermedia is lower than most people think, particularly with compression:

https://github.com/1cg/html-json-size-comparison

in most real world systems i expect data store access to be the dominant latency issue

some people have had good success w/htmx from a performance perspective (2x+ speed, 1/2 memory consumption):

https://htmx.org/essays/a-real-world-react-to-htmx-port/

but of course it depends on your use case.

I think htmx would be excellent for an e-commerce site, which is mostly text and images and relatively simple, coarse-grained user interactions.

That must explain why websites are so fast and responsive these days! /s

If you're wedded to the idea of dynamic reloads, which - imho - you ought not be, HTMX or Turbo are ways of achieving that without needing to write any JS. HTMX is designed to complete HTML as a hypermedia language, which it does using JS, but it itself does not require any additional JS to use. I'm sure the authors would dearly love it if HTMX's ideas were taken directly into HTML5, and they could scrap the JS library altogether. Its use of JS is an implementation detail borne of necessity.

If you're coming from SPAs and can't imagine websites working any other way, I highly recommend reading something like Hypermedia Systems (https://hypermedia.systems/). Come with an open mind, let it be your gateway drug. HTMX builds on a very old tradition of hypermedia, all the way back to Computer Lib/Dream Machines, through systems like HyperCard, to the actual, original definition of RESTfulness (that modern Ajax SPAs generally do not meet). It draws deep from the hacker ethos (you know, that thing that this website is about!).

Personally, I don't even think HTMX is all that necessary. What I feel like you're missing is that the overwhelming majority of websites are technically very simple, and their value to their users is in their contents, not in their stack. Did you notice the full page reload when you hit Reply on your comment, here on HN? Were you terribly put out by it? Would HN be an improved website were it full of spinners and Ajax? I would respectfully suggest it would not. It delivers amazing value running as a Lisp program on a machine under someone's desk.