Most websites built as SPAs using React are slower, buggier and more expensive to maintain than if they had been built using mostly server-side rendered pages, like we did in the 90s.
Most websites shouldn't be built with React. Most single-story buildings shouldn't be built out of steel I beams. That doesn't make them a bad material, it just means you shouldn't use them if a simpler material will do.
But many are! This is the issue. 10-50x the effort, complexity, hours, expense. Lots and lots of steel, everywhere, because managers heard that Megacorp uses steel and now insist on it.
Steel beams everywhere.
I don't use React because I'm being forced to. We chose React!
For websites: we all like to dump on PHP but it was literally designed to build websites with relatively limited features / large reach - which covers a large swathe of the web.
So SPA: good for ERPs, CRMs etc but bad for webshops, social networks, TODO apps, news sites, Streaming sites, blogs etc.
[Why Angular? React is technically interesting, especially that it's build around the Rx pattern popularized in dotnet, however I have a deep-seated hate for Facebook for their key part in making Brexit happen which trumps any technical merit. Vue wasn't a thing back then]
We are slowly incorporating Vue into some aspects of the application and the code is just so much cleaner and legible, with better performance in a lot of cases.
Once SPA has been loaded, it switches to SPA internal pages faster than "like we did in the 90s" because for static pages there is no network round trip and no delay called TTFB. For dynamic pages, making API call and fetching API data (to use it for CSR) likely takes less network bandwidth than fetching HTML generated on the server (as a part of SSR).
Also server rendering uses server CPU and it's never totally free. Especially if server side needs to be scalable.