The important thing is that client-side rendering stays on the client and doesn't need to go back to the server.
It still needs to go back to the server to fetch new data. And it's not like rendering on the client side is instant and free. There are plenty of JS-heavy websites that visibly lag during UI operations because of all the stuff that goes on during "rendering" (in quotes, because it usually includes large chunks of business logic).
If you want to use a decent library, you can rest assured that your client-side templates will render far faster than the Ruby version of the same HTML.
Even considering I do not have an 8 core machine with 32 GB of ram sitting on my lap (or in my pocket)?
I realize that ruby is significantly slower than v8/JägerMonkey/Tracemonkey/etc, but is it so easy to discount the significant disparity between the average compute power of a server vs mobile/laptop?
I think a stronger counter argument would be flexibility, smaller http responses (and thus less latency), and possibly an argument that it is simpler or more straightfoward, in favor of client-side javascript templating/rendering, but rendering speed? Not so sure.
It still needs to go back to the server to fetch new data. And it's not like rendering on the client side is instant and free. There are plenty of JS-heavy websites that visibly lag during UI operations because of all the stuff that goes on during "rendering" (in quotes, because it usually includes large chunks of business logic).