Internet browsing was also regarded as a light activity back then.
Nah. Web browsing was big back then for most, as well.
But the change to which you speak, of sites being slower on older gear, is for one reason alone: it isn’t the computers.
It’s for all the tracking-based advertising and heavy javascript/XHR weighing down the web sites themselves. Also, the cookies associated with that ad-tracking is a player here (even if cookies themselves are tiny strings).
Some javascript advances were meant to make loading a page faster by incorporating a “progressive loading” feature (which can reveal itself as one scrolls down a just-opened page), but these advances were incorporated into later iterations of javascript. Other, more recent javascript functions, many of which are designed to “app’ify” a page (where XHR steps in), weigh down substantially the loading and functionality of the page.
I used to think responsive web site coding was a major factor. My thoughts changed when I re-built my web site in 2013 and I heavily modified a responsive theme for WordPress.
When done well, though, responsive-aware javascript doesn’t weigh down the loading of a page on an older system nearly so much as embedded javascript and XHR-related functions used for ad tracking and
app’ifying, respectively.
Anyway, I needed to get this bee out of my bonnet. Also, using uBlock Origin legacy plus uMatrix legacy, and making browser-level optimizations, can and do make older systems move nearly as quickly on many web sites as in, say, 2008 — just so long as those sites aren’t heavily relying on more recent JS/XHR (a good example, paradoxically:
iFixit’s web site).