> If it needs to fetch data from your API server to render, so long as both servers are in the same data center, latency should be very low

Maybe I'm just missing something obvious, but wouldn't FastBoot make things slower if it needs to go hit the database to produce the static page, and then the clients hits the server again to fetch the unrendered templates and data as part of Ember's normal bootstrapping? I don't think it's that uncommon for the database to be a bigger bottleneck than the network.

Are there any benefits over, say, slapping a screenshot of the page as the background of body? (yes, I know I'm oversimplifying, but for instance, Facebook loads up wireframy graphics as placeholders until ajaxed stuff pops up.)