Basically, JS wasn't leveraging much of the computer's power until V8 came along.
Having written significant pieces of code in javascript over the last 13 years, I've benefitted massively from those speed increases, and while I'm aware of a number of reasons that they have occurred (including but not limited to the v8 engine - I remember reading excitedly about tamarin, tracemonkey and the like), I don't think it's silly to point out the sheer magnitude of the improvement.
I'm not trying to say that JS is going to save us from Wirth's law or anything, I'm just trying to give people context on what a huge jump it has been.
By using JavaScript you are, out of the box, sacrificing about 20% of your performance for the comfort of using it. In some cases, maybe only 10%. In others 50%.
I can totally understand how that's a trade-off one is willing to make in many cases. But for desktop apps, I kinda want them to run at full speed. Like, a 0.2 second delay can get annoying if it comes at the wrong time or stacks with something else. And somehow, you can feel that in many hyper-modern apps that are so many layers removed from the hardware, the devs probably don't even know where it touches it.