So in other words: the main reason removing the React code in the frontend in this case gave such an immense performance benefit is that the logic used to compute the rendered UI is significantly more complex than the logic necessary to make that UI interactive.
It's not React that's slow, it's the logic needed to render the page?
That's pretty close. For our use case (and many others), the biggest culprit for increased TTI is the sheer size of the JavaScript payload and the time the browser takes to parse the whole bundle[1]. We still use React and other libraries on the server to generate the HTML but rather than sending all that JS down, Tony worked out exactly which client interactions still needed JS and wrote those in plain JS which resulted in a big TTI win as the browser simply had less data to parse.
It's not React that's slow, it's the logic needed to render the page?