Number one is being careful about NEVER triggering a repaint while an animation is happening. That means only animating css transform properties and opacity, and deferring anything that updates the contents of an element until after an animation is finished.

Number two is that, even though it's running a physics simulation for each animation, we kept the physics code really simple. The simulation code generally takes less time than the drawing itself.

It also doesn't rely on jquery animate, which has a lot of extra cruft to run on older browsers and support legacy code. Writing the rendering in pure javascript helps a ton.

And I think the last thing is that, when the animations respond with the velocity of your interaction it makes everything feel really smooth.

Even if an animation runs at 60fps, if the way it responds to your touch feels bad, the animation won't look good.