User blog comment:Rappy 4187/Technical Update: January 18, 2016/@comment-1370845-20160119002404/@comment-126761-20160119004608

We've been trying out a few different types of notifications recently, to see how well they work. While it's possible we'll use something like this again in the future, we want to avoid leaving notification types around longer than necessary to avoid information overload (which is also why we retired the notice in the classic editor preview dialogue recently).

ImportJS is mostly intended as a quality of life tweak, rather than about site performance.