The performance difference is almost certainly negligible, but by having one html file instead of the seven, you get a smoother ux by changing only what's different with javascript (no page refresh, no flicker).
If you want to stick to vanilla js or jquery, put all the html in one file and toggle the elements' css display properties.
A cleaner and easier way, though, is to use a framework like Angular. That way you can break the dynamic elements into partials, and you'll end up with eight clean, concise html pages (total html = one page option), and need no javascript at all. The best implementation depends on your specifics, a simple one might look something like:
<!DOCTYPE html>
<html ng-app>
...
<body ng-init="partial = 'initial.html'>
<!-- header, whatever part of that 20% -->
<!-- put dynamic elements in separate html files (no html tags or anything, just the divs or whatever -->
<a ng-click="partial = '/path/something.html'">something</a>
<a ng-click="partial = '/path/whatever.html'">whatever</a>
...
<div ng-include="partial">
<!-- rest of that 20%, footer -->
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.3.15/angular.min.js"></script>
</body>
</html>
I like Angular a lot. It's incredibly powerful, yet you can use it for super small things like this with just one script
tag. (If you want the back/forward buttons to work, look into ng-route.) Either way, your presentation will be a lot more impressive if you don't use seven static html files. Cheers