SPA loading performance can be improved by splitting React app into multiple SPAs each rendered by its own smaller bundle. Additionally, for many websites, the landing/index page of a SPA can be prerendered at the build time. The result is the best performance ever achievable for many (but not all) websites.
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.
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.