I guess having static pages is the most scalable in a CPU power sense, maybe to get over the navigation / update problems you could build a PHP script that refreshes from a database once every 12 hours or so via cron, so any new content or navigation links auto update with time and produces new HTML pages.
To answer the OP's question, yes I think a static page is going to be as fast as you're going to get. You can still introduce performance issues with bloated JS, CSS or a lot of media but that's won't effect the cpu load.
On a practial note, you can build a highly scalable CMS by using caching, compression and resource combination techniques; as suggested above you would probably be a lot better off in the long run.
In the end it's all HTML with links to other pieces of content (images, css, etc)- Just consider the practical ways of getting ther and whether or not it's work the hassle.
Static page is scalable as far as serving them is considered, but without some sort of scripting/processing you won't be able to generate user contributed content.
You can overcome the navigational and other related issues, but still to do that you will need some sort of processing either on the server or on your PC, in practicality you need computing power, memory, scripting, database etc. etc. somewhere.