Choose the number of dots you'd like to animate and then choose the engine and click the "START" button below. Watch the fps in the lower right (you want that number high - it is the total average frames per second rendered). As more dots are animated, you'll see the performance gap widen. Try to push things until the fps drops below 30fps. When the CPU isn't breaking a sweat, fps should hover around 100fps in most modern browsers.
Note: Zepto uses CSS3 transitions which won't work in some browsers (like IE). Also beware that some browsers incorrectly fire requestAnimationFrame events even when the browser clearly isn't updating the screen and/or they handle JS in a different thread, thus 10fps transitions may inaccurately report in JS as running at 50fps. So when you're watching Zepto animate, don't pay much attention to the fps counter because it may be artificially high. Instead, watch the actual animation of the starfield and how smooth it is (or not).