I’d rather have sluggish UI with proper feedback than potentially inconsistent states which I often experience with modern SPAs. At least that represents reality. Just today I was posting an ad on the local classifieds page, and the visual (client) state convinced me that everything was fast and my photos are uploaded. Turned out all state was lost and never reached the server, and I had to redo everything again.

It’s trivial to achieve under 100ms in the US with even just one server.

Most companies aren’t international.

Without keep-alive, any HTTPS request requires multiple round trips to complete.

Fortunately every browser made in the last 25 years supports keepalive. e.g. Firefox (and according the the reporter of this bug, Chrome) won't even let you disable it[0].

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=879002

Barring the interactivity SPAs will also end up talking to server anyway. So even SPAs will feel sluggish in a high latency env.

Phoenix Liveview works pretty good without client side state. Sure if you just have a toggle mobile menu you might sprinkle some JS for it but other state lives in the server and the delta is sent to the client via websockets