Improving Performance on Twitter.com
engineering.twitter.comThey are geniuses... who would have thought that rendering HTML on the server might be faster than loading up a page then having JavaScript request the server again for the data then render it?
To be fair, the previous version included the JSON data for the page alongside the JavaScript in the initial payload, so it wasn't waiting for a second (redundant) data fetch (for the main home timeline page that's cited in the article as seeing gains).