Back at the beginning of 2014 I was having a debate (I used air-quotes there people) with some fellow developers about the irrelevance, or not, of worrying about CSS selector speed.
Whenever exchanging theories/evidence about the relative speed of CSS selectors, developers often reference Steve Souders (http://stevesouders.com/) work on CSS selectors from 2009. It's used to validate claims such as attribute selectors are slow or pseudo selectors are slow.
For the last few years, I've felt these kinds of things just weren't worth worrying about. The sound-bite I have been wheeling out for years is:
With CSS, architecture is outside the braces; performance is inside
But besides referencing Nicole Sullivan's later ...