> That is more processing

Not necessarily. Often it is less. Template engines can be very, very efficient and fast.

Returning JSON almost always requires data/object serialization on the server, this is often slower than direct data/object -> template rendering.

Further, it's not your server but keep in mind the client must de-serialize JSON and render some HTML every time.

Modifying layouts as a result of non-persistent state (light/dark mode, sorting, etc) can usually be handled relatively easily with styles and data- attributes and sprinkles of JS. HTMX works very well with Alpine.JS and similar libraries for more complex situations.

HTMX isn't for every scenario, but it works very very well in many scenarios.