For the same data, why should serializing it to JSON be particularly faster than rendering it as HTML? The server is converting the data directly to a byte string in either case, which is about the same amount of work.

HTML is more verbose, so I would guess that JSON serialization is slightly faster, but I doubt there's an order of magnitude difference. (I could be proven wrong though)

I agree that taking HTMX to the extreme where _all_ interactions require a request to the server is too much overhead for interactive web apps. But there's likely a good middle ground where I can have mainly server side rendering of HTML fragments with a small amount of client side state/code that doesn't incur particularly more or less server load.

Converting data to a HTML string is not a performance bottleneck you'll be worrying about. I wasn't worrying about it much in 2000, and you really shouldn't need to in 2023.