> PHP's performance can be significantly lower than JS, because it doesn't have application state (in a standard runtime/setup) and needs to re-run the entire application for every request.

Sure, in PHP, the reality is that after your request is processed, all the state is garbage and is thrown out. But once you embrace that reality and stop trying to make sculpture from garbage, you can make some pretty damn fast pages that get straight to the point. Of course, a lot of people look at my fast PHP and say that it too is garbage, but at least it's fast garbage :P