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.