> but developers "fixate" on it because it matters to users

Then it's not a fixation. If, for your application, the difference between a 1-50ms interaction and a 150-400ms interaction matters, then you have reason to for that interaction optimize the performance. If your entire app is like that, then you're probably a particular type of app (superhuman, miro, linear, etc.). Don't get me wrong, I pay for superhuman because it feels like a native app and gets many things right. I would not pay for hey.com. But hey.com is actually relatively successful out there despite its sometimes-perceptible latency. Oh, and superhuman has perceptible latency too. Any time something needs to come from the server it takes that round trip time regardless whether or not that thing is JSON or HTML. The delta between those two is pretty small now-a-days, and typically not perceivable.

> If you haven't got state that affects the UI, maybe you're the unicorn. I've found that websites always need that kind of thing (tabs, radios, nested dropdowns) and once you have something like that you either have the state only on the server and the latency is high enough to bother users

I mean, we do. Our forms are pretty sophisticated. Things add/remove show/hide, and they update live as others make changes. The state just doesn't need to be on the client. You say maybe we're a unicorn, I say maybe people should consider whether or not they are a unicorn too, and maybe those unicorns are just horses developers think are unicorns. They may just be a lot more common than you (and others) think. We all like to think that latency matters to our app because we all read that article saying that Xms in latency cost $Y, but none of us stopped to ask if that article was about our app.

> But you don't need to know the list of tags or attributes or the escaping rules or the rules about which tags are self-closing and which aren't or ... . You just use React components and follow their documentation.

I'm not sure how to say this... but literally none of this is typically significant and certainly not any more significant than it is in React. In React you still need to know the tags and the attributes. Escaping rules? Yea, you need to know those too -- just the JavaScript ones. Self-closing tags? Just don't use them, or do if you know them, or use something like HAML/Slim/any other templating language to do away with those nuances. I'm not sure that arguing that HTML is hard to learn is a very effective platform. It's the thing that grade school kids learn to make web pages. Sure, there's edge cases, and understanding semantics is hard, but you don't need any of that.

> I suspect the average React app probably is slower, partly because React sites are generally newer...

This isn't why. It's because it requires a significant amount of JavaScript to load and evaluate before you see anything. That amount can get larger with time.

> It's O(1) work though, or very close to it. I was the guy that used the existing build that someone else had come up with before I started working on that codebase...

Right. That stuff is O(1)-ish, and maybe it's stabilized some, but having gone through the various transitions, I can say it's not free. Nothing is though, upgrading Rails took us a couple hours this time around because of a breaking change for our 30 apps. React comes with, as a baseline, significant complexity that is O(n): Client/server separation (APIs or phantom-APIs like Next.JS has), state entangled with presentation (See the whole smart vs dumb components for an attempt to address this), massive asynchronous concerns that cannot be fully abstracted away (suspense and the like), and probably more.

Folks can't see it until they step away from it and look back. I've done that for the last 3 years. Unless a person has done it, I wouldn't expect my arguments to land. Folks see things or they don't. Again, I'm only here to say: there's something to see that you may not see yet. I'm sorry I don't have anything more convincing.