Javascript doesn't affect page load (generally). What Phenomic (and my project Gatsby) speed up is clicking around within a site after the initial page load as we pre-cache subsequent pages so loading those pages is near-instant.
But in any case, client-side routing to me is a nice-to-have not the killer feature for Gatsby. Building web sites with the React component model is the killer feature for me.
FWIW, while I'm in the US so don't really understand developing for very poor networks, the methods I'm describing is exactly what companies in India and other places with poor networks are adopting: https://developers.google.com/web/showcase/2016/flipkart
This is also a good read https://developers.google.com/web/fundamentals/performance/p...