I think the fact that animations are occurring right away is what contributes to the illusion. Most sites with any kind of animation aren't using CSS for it yet

I think the fact that animations are occurring right away is what contributes to the illusion. Most sites with any kind of animation aren't using CSS for it yet, and on those sites the javascript (which usually begins execution late in the rendering) introduces extra time to load any kind of animation, leading to a perception of slowness that is actually occurring client side.