Ask HN: Why is the web becoming so resource intensive?

I recently dug out a few old laptops. After installing lightweight versions of Linux, they worked absolutely fine and at a speed not noticeably slower than my daily MacBook Air - but for when I tried to use the internet, where only a few sites worked.

Why is that happening? It seems to me not necessary to have such heavy sites, and from what I could see, it was mostly the UI, or otherwise nonessential elements, causing the bloat.

As a bonus question, are there any low resource sites that you use (for instance, this)?