I would like to know more about what was causing slowness than those couple of bullet points. I've only made a few websites over the years, and they were always serving semantic HTML and CSS; what scripts there were granted interactivity and weren't necessary to view the page. So is this a matter of people turning mostly-static websites into React monstrosities or is it something else?

Besides respecting sane HTML practices, the point about trying to get info that is displayed on a map is interesting. There is no reason that I can think of that that kind of public information shouldn't be freely available as an API, so that anyone can build a frontend for it - including a plain HTML <ul> frontend. That kind of thing should really be mandatory for public services, because it maximises the utility of said service. Ideally it would also be mandatory to include some more-or-less raw display of the API data so that no third party needs to build a frontend, but start small.