Yes, the runtime is slow. But many language design decision, notably references, make it difficult to make a fast implementation (I should know, I did my PhD on it).

They're trying to explain the problems that Facebook are having, which is that there is just such a large volume of code. They aren't trying to give a short course in interpretation and compilation.

Scripting language is not a meaningless expression. Many papers use it. I submitted papers with "scripting language" in the title to PLDI and POPL, and there was not one complaint or problem with the term from reviewers (who found plenty of tiny "errors" otherwise).