We used languages like PHP/Ruby to produce HTML/CSS that browser renders. Then Microsoft introduced XHR. Then we started with "saving bandwith" and "page does not to be reloaded" and AJAX was born (although it was Asynchronous HTTP and HTML instead of XML). Then such sites weren't indexed by Google so we started to pre-render content again for the most visited URLs.
And now there's so many buzzwords and bland excuses about server side rendering, such as "SSR improves user experience: users don't have to wait for CPU-bound JavaScript to run on the client".
JavaScript itself is not what makes websites slow. It's the 50 megabytes of advertising material I've to load from various "content delivery networks" that are everything except fast and auto-play videos that I'm not interested in.
Today's personal computers are stupidly fast, smartphones too. There's so little gain in the argument that pre-rendering the first request on the server improves performance while it does nothing for the network-related problems (resolving the domains and transferring so much content).
SSR is great, don't get me wrong, but the reasoning that encourages devs to use it is misplaced (not WRONG, misplaced).
Yes, it does improve user experience, but it doesn't do so because our computers are slower in parsing and executing JavaScript.