A HTMX-y app will typically be a fetch-heavy app. Think TODO list, where each time you add a todo, you want that persisted on the server anyway. Yes you are sending more down the wire. I doubt rendering a partial HTML template is that much more work than formatting JSON but there is more crap to send down the line for sure.

Where the application is doing lots of micro-level stuff, like maybe you are drawing on a canvas and occasionally sending a payload of updates to a server, I think then HTMX would be much slower and clunkier, and that is when you dust off your React.