Oddly, it use 100% of one of my i7 iMac core when running on Chrome (6.0.408.1). However, the animation play very smooth with only 1~2% CPU usage when running on Safari (4.0.5 6531.22.7).

So I assume you're using Chrome?

Edit: -webkit-animation in general seems to be very slow in Chrome.

Edit again: Oh right, -webkit-animation is hardware-accelerated in Safari. http://webkit.org/blog/324/css-animation-2/#comment-25071

Huh. I have a new 13" Macbook Pro and my CPU is at 5%-8% in Safari.

However Google Chrome does not look like it hardware accelerates CSS animations so it's eating up 90% CPU on this.

Odd. Desktop iMac, older Core 2 version, it only takes me from my background 2% cpu use to 7% cpu use. Perhaps it is making heavy use of the GPU, which I wouldn't see in the cpu usage stats. (I add: using Safari.)

Odd. My 2.26ghz 13" MB Pro puts Safari at under 10% CPU usage for this; the CPU doesn't even clock over 1ghz running it.