A few simple questions about javascript

I work in a small web design studio. Once again, watching the many days of torment of our programmers who are desperately trying to implement the idea conceived by the designer in JavaScript (after all, as usual, you need to work everywhere, so that it looks aesthetically pleasing and does not slow down). I have a simple question ...

And why does he actually slow down? No, I can understand why large computing power requires digitizing video in real time, or for example, why we need powerful processors for rendering complex 3D scenes ... I almost resigned to the fact that with the release of the new version of my favorite shooter, my old video card can be carried to the dump :) ... I somehow can explain all this logically.

But I desperately do not understand why browser vendors give out a 20-30% increase in javascript performance for some unreal breakthrough? Please explain to me why such a problem arises in principle? How does javascript load so heavily an average modern processor, literally putting it on its knees? After all, the idea of ​​these gigahertz should be more than enough for not complicated animation and simple effects.