Yes it does take too many CPU cycles but only if you are running a planet scale app like facebook/twitter etc, even 0.001 extra second of compute time could cost hundreds of thousands of dollars for facebook billions of users.
>server side rendering
AFIAK that just renders out the initial data of the SPA rather than loading it from json the first time, just to speed up the app. Thats fine but it doesn't handle updates to the app (ex: scrolling to the next batch of tweets on twitter). Its still an SPA.
>Twitter doesn't offer much more interactions than an imageboard does.
thats true but the original version of twitter was written as a server side rails app that they rewrote to spa+scala for scalability.
JSX is only for react, angular components are totally different