Anyway, so problem currently is that client devices are slow and HTTP requests are expensive/slow. If these were not issues - I guess rendering on a client would be just fine?
If above is correct, I do not see good reason for smallish websites/apps/whateverucallit fallback on server rendering, as it looks like every single year mobile devices getting faster and faster. And we have SPDY on a way to mainstream.
Do I want to wait 3 seconds to get HTML or wait 100ms to get bootstrap code and see pieces rendering as data comes in? Probably doesn't matter, since it will take roughly same 3 seconds to render it on client side.
Ultimately, perceived responsiveness of webapp depends a lot on particular implementation. If you wait for all data required for page to render - yes, it will be slow. If you render pieces of page as data comes in - user sees that something going on and this is good enough. Why it should matter where data will be converted to presentation, on server or on a client? It is still waiting. And if client is not happy with it? Buy beefier hardware!