I think these rules (and I think they are excellent) are poorly named as "efficient" css. That makes me think of performance.

In that case, yes it's less performant. But the specificity problem is so significant that I've followed the rule of never using ids for styling at all. The problems I've run into with CSS have entirely been regarding maintainability and preventing visual bugs, I've never had any CSS performance problem worth addressing. (I almost skipped this submission because I thought that was what is about, I'm glad I didn't.)