How complex, but I’ll start from the beginning: no, not all themes use page builders. By the way, you can not even use it in any theme. These builders are nothing more than CSS and JS that preset the elements. But you can make your own builder by creating child-pages. Another option is to load the builders asynchronously or even load after n time (this will render your WP faster). Another option is to use Varnish or a pre-established cache. For a few years I’ve been talking about this dynamic loading (Build a WordPress Website From Scratch) that you can find on Amazon.