Well, "several orders of magnitude" is a bit much, but the point stands.
However, that's only during the initial load. After that, JS can just keep modifying the DOM based on the data retrieved from API, and never download HTML and construct new DOM again. If done properly (and that's a big if!), and where appropriate, this can be much faster.
> Most web devs develop on localhost and test on some of the best connections you can get today, leaving network performance testing as an afterthought at best... and it shows.
Very true! And on beefeir CPUs/GPUs, more RAM, faster storage etc.
For the last couple of years, I've been careful to develop on "midrange" hardware, exactly so I can spot performance problems earlier.