37signals (now Basecamp) should have a post or two about how they made a new version of the product fast. One of the points was bringing templates back to the server, iirc.
It is usually the loading of many resources (js etc) that takes time. If you just send some html (and css) to the browser it can show it quickly. But companies like Facebook and Airbnb are starting to send html first and then use javascript for everything. This makes the first pageview very fast while still keeping the flexibility and other good parts of a SPA system.
