Don't forget that JavaScript has to be interpreted every time the page loads, even if you have a cached copy of it. If it's a large chunk of code, the time to do that is not trivial.

I'm not quite sure that it's as much of a zero-sum game as you present it. I can easily think of scenarios where rendering on the server is much faster (e.g. using a compiled language vs JS, taking advantage of powerful hardware, granular caching, etc) and much more constant.

Edit: spelling.