But the page load time is not the optimization objective. What really matters is the time that it takes for the page to become usable, in other words - when users can start consuming the content on the page.
The first iPhone was launched about 2 years after the original AJAX piece was published. And that triggered a new epoch of mobile browsers. But mobile browser CPU time is not free. It doesn’t cost money directly. The price is the user’s disappointment with battery life. Now users don’t blame lazy developers who wrote suboptimal code, they blame mobile device vendors for using bad batteries.
A crazy mix of server-rendered HTML pages and lots of jQuery based mostly on spaghetti code to update these pages, plus a few JSON/XML APIs here and there, was characteristic of Web 2.0 development for many years. By that time, the prestige of frontend development had significantly declined. Backend engineers looked down on HTML/CSS/JS folks that spent most of their time showing and hiding divs and changing the colors of different elements.
The state of frontend development was now where backend development had been in the late 1990s - a zoo of approaches and technologies. Different groups tried to come up with a frontend framework that would straighten out the mess. Jeremy Ashkenas came up with the most successful one at the time, Backbone.JS. Released in 2010, this client-side MVC framework restored some order to jQuery-based frontend apps. A few years later, Marionette came along to attempt to enhance the functionality of BackboneJS and to become a Rails-style opinionated frontend framework.
Then more and more engineers began to develop frontend-based apps. And the fun was about to begin.