Clientside rendering doesn't need to be heavy at all. In fact, you could do it with just $.getJSON('/api/users', function(data) { $('#users').text(data.content) }.
Sure, that requires jQuery, but most "normal" sites require jQuery, too.
And now your poor little mobile user has to wait for the page to download, then to execute javascript, then to wait for the API response, then wait for the DOM to update.
Or you could put it in the HTML, and then they just have to wait for the page to download.
For each new page on your site that your user loads, the benefit of single page apps becomes greater: Now she only has to load a bit of JSON, not a full page. So essentially single page apps are a tiny bit slower on the first page load, but much quicker on subsequent page loads.
I'm suggesting the hybrid approach here, exemplified by the Rendr and React.
Give the full html of the first page, load the JS necessary. You get the best of both worlds.
On the first request, the client receives a rendered html page, gets to read the page, then JS functionality is attached.
On any subsequent pages, you do a simple JSON request and update the DOM.
The approaches come together without sacrificing anything. You get to write a single code base using JS (with a node.js backend doing string manipulation so it doesn't even need a DOM), the user gets a page without having to wait for external resource loads, and all page transitions past that are a simple data-fetch away.
Sure, that requires jQuery, but most "normal" sites require jQuery, too.