It’s much faster than that in practice, but of course it comes down to how well your backend is written. I’ve been using HTMX lately and I can blink and miss the UI updates.
I wish I had numbers, but in my experience it’s far better than you’d expect. Basically take the length of a REST call you’d have to make anyway and add a few milliseconds for the rendering.
It won’t be the right choice in all cases, but it’s a great option in many.