Rendering anything but the simplest DOM templates from JSON can be extremely slow. We so wrote a book on JavaScript performance -- runtime performance, not simply asset delivery. In that nearly 4yo book, we recommended against templating. A couple years later, we thought things got better & faster and it wasn't an issue any more. We thought wrong.
It is usually more than fast enough. The project I'm currently working on renders completely on the client with around ten different templates/partials, and the rendering adds up to just a couple ms - that's using handlebars which is not the fastest engine around. 99% of the application time is on data retrieval and processing.
I understand you had issues with your project, and email apps are complicated beasts, but let's not generalize. The truth is in between. I bet rendering a complex template server-side on Ruby can be slower than js.
