Getting that JavaScript is a one-time operation, and a 304 from then on. Also, the HTML can include bootstrapped data, saving the roundtrip for JSON.

Also, with client-side rendering you execute more code on the client but less on the server, so in an environment like Twitter where it's not possible to do any sort of heavy caching (everybody sees something else), you're simply trading time on the server for time on the client. Not faster, not slower.

Server-side HTML generation is not a magical 0ms process.