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.
http://jsperf.com/dom-vs-innerhtml-based-templating/350
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.