We’ve been adding mountains of JS to “feel” fast, while making everything slower.

I disagree that that is a bad thing. I’m not providing evidence here, so disagree if you wish. In my opinion, users don’t care if something is fast. They don’t take out stopwatches to time the page transitions. They care if something feels fast. They want the web page to react immediately when they issue an action, and have the impression that they’re not waiting for too long. Feeling fast is way more important than being fast, even if the feeling comes at a performance hit.

It takes about 120ms for a human to detect (not react to) a stimulus [1] (For the gamers: That’s 8FPS). So if you page responds to a user action in that time frame, it feels instantaneous.

If you want to try it yourself, paste this code into your browser console. Then click anywhere on the page and see if the delay of 125ms feels annoying to you.

let isBlack = true;
document.body.addEventListener('mousedown', () => {
  setTimeout(() => {
    document.body.style.backgroundColor = isBlack ? 'red' : 'black'
    isBlack = !isBlack
  }, 125)
})

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC4374455/