To sum it up, you shouldn’t worry about selector performance, unless you really go overboard. While the topic was all the rage in 2012, browsers have gotten a lot faster and smarter since. Even Google doesn’t worry about it anymore. If you check out Google’s Page Speed Insights page, you won’t see the rule “Use efficient CSS selectors” which was removed in 2013.
Instead, focus on making your CSS maintainable and readable. Your colleagues and your future self will thank you for it.
I am really into utilitiy-classes in these days. Tailwind CSS and even Bootstrap 4 is embracing this.