Like others have said, major Javascript engines are much than Ruby implementations, owing to the huge effort Firefox, Google, Apple, and Microsoft have put into those engines. In principle, I'm not sure if there's reasons that make one intrinsically harder to optimize.

But even if Javascript were slower, it wouldn't mean the prediction was right. Javascript didn't do something super amazing that let you write beautiful code (the way Rubyists feel about Ruby, or Python folks feel about Python). It just ran the web so we used it. Over time, it's morphed into a language that's about as expressive as any other good dynamic language, also with some warts. But it's not a leap ahead, which is what the original post predicted.

The only language I've felt that way about was Prolog. When it works right, it's amazing. Too bad I can rarely make that happen.