I've been using the Genesis Framework to build custom themes along with Visual Builder tools like WPBakery Page Builder. The more I learn about file size and website performance, the less I want to rely on these tools. They seem bloated and opinionated.
Personally, I would like to build light-weight, highly effective websites using some kind of CSS framework along with a bare bones WP theme. The problem is, I don't really know where to begin. With the goal of building fast, light-weight and highly customizable websites, which CSS framework and "base" theme would you recommend?