Agreed, speed matters if it as you said comes with a more efficient implementation that is far more developer friendly. With ES6 and soon ES7, notably Object.observe, we may be getting the best of both worlds. However, in terms of speed for jsblocks I do not see the server-side rendering feature as a plus or really even a feature.
Yes. ES6/ES7 are great but they are not ready yet. I am looking into ways to make jsblocks more ES6/7 friendly. Regarding the server-side rendering I believe it is very powerful feature. We use client-side framework to improve user experience and server-side rendering does improve user experience because there is no flickering before the framework kicks in. And if you cache the pages you will also get extra performance boost.