See url test page, when trying to pan on that page, the page scrolls rather slowly and stuttery. In Chrome on Android, the scrolling is smooth.
Thanks for the profile, it confirms my suspicion that the stutter is from synchronous painting and that async subframe scrolling should fix it.
APZ -> Nightly+
Nothing to do here until we get APZ working on fennec.
This works well with APZ on nightly. Making this depend on the bug that tracks shipping APZ on fennec.