Considering that it runs very smooth for me at 1080p using a first generation, low-end quad core and ultra cheap onboard video, I can only assume that you either have a technical issue with your system or you're using a browser that doesn't properly support some of the required CSS3 features.
It's not enough to say that it runs smoothly for most of the animation. It should run smoothly for virtually every second of the animation. I have a computer that beats yours hardware-wise running Chrome, and there were many places in the animation where fps dropped below an acceptable threshold. Some people are also more trained to notice fps drops too.
Running "very smooth for you" doesn't mean much. For one, it doesn't do much anyway -- some circles, some boxes, some bordes, shape tweening, and a soundtrack. You could do this stuff in 1080p in 10 year old hardware if you did it native. Heck, you could probably even do it in an Amiga 3000 circa 1992. I sure remember an O2 (I think it was called) by SGI doing far more impressive real time stuff in 1996. And that machine had about 1/5 the capabilities of my iPhone.
The thing is, it uses quite a lot CPU power for what it does (and no, it's not "technical issues" with individual systems: it tends to do so in general in every system), and the animation isn't always smooth and far from 60fps.
And the suggestion that he's "using a browser that doesn't properly support some of the required CSS3 features" only reinforces his point, that when it comes to web technology it's a large step backwards from what we can achieve in native systems.
