You're probably comparing htmx with client-side framework like React. Unlike those client-side ui framework, htmx is used to enhance standard html pages, such as those outputted by server-side web framework. Those frameworks typically renders detail and form page separately, and htmx can combine them into a single interactive page. For example, when you reply a comment in HN, it'll open the form as a new page, and when you submit the comment, it open another page. htmx can combine all of those pages into a single interactive page.
Should be a bit faster because it only requests a piece of html fragment which then get swapped into the dom instead of reloading the whole page again.
Yes. That's why I specified "simple page loads". If your frameworkless website is slow then the fault doesn't lie with the nature of http or the intrinsic overhead of full page loads. You've got a shitty backend framework, database or network.
It's already supported with the client-side-templates extension [1]. HTMX can fetch JSON from the backend then calls e.g. nunjucks.render() with the data.
Not at all, you can preload as many data as you wish and use vanilla or a companion JS framework like Alpine.js or HyperScript or any web-component to make the current page interactive without fetching the backend. It's a trade-off between the initial page size vs. additional network delay.