On the bright side, the performance tax we had to pay before the current generation of frameworks was often the "full page reload" tax, needing to parse megabytes of CSS, JS for almost every user interaction.
If you haven't already figured out, everything old is new again. This was an old pattern when someone taught it to me at my first job. It felt like it wasn't happening for a few years but only because everything was new to me. Then I started seeing people win arguments arguing for the things that we had just finished replacing, and the most I've ever seen since is brief respites as the pendulum approaches the apex on each swing.
Old-school AJAX shipped HTML to be dumped straight into the document. I suspect that was much faster than the modern approach of sending, typically, JSON, which then requires deserialization, a bunch of transforming, memory allocation, shifting data around, template application, et c before any DOM changes are handed over to the browser engine, at which point you've just finally caught up with the approach of shoving HTML into the DOM directly.