> A server side website has to bring down the 20K page again and again as the user navigates through your site. Caching or web server configuration makes no difference here.
Browser caching, not server-side caching. The browser doesn't have to "bring down" the 20k page "again and again", it gets the assets out of its cache (html, js, css, images, etc), and then the JavaScript loads the 5k article when the DOM is ready. Hope you're not trolling. Non-SPA websites can be basically just as fast as SPA's, and in practice they are often faster since they are usually built without frameworks and have less JavaScripts.