Try adding -webkit-overflow-scrolling: touch; to enable elastic/momentum scrolling on the .parallax div.

EDIT: Nevermind, just tried it in the simulator and it seems to break everything. Although it is smooth... :)

There's no JavaScript on that page outside of Google Analytics. You're right though, it feels pretty terrible without the native scrolling momentum.

Works OK on Firefox OS. The base layer scrolls perfectly, but the other ones update at a lower framerate. Desktop Firefox has the same behavior when APZ is enabled.