STR: 1. Run the latest nightly on Linux. 2. In about:config, set layers.async-pan-zoom.enabled to true. 3. Load http://people.mozilla.org/~bballo/grid.html. 4. Scroll with the mouse wheel. Expected results: The page scrolls at the same speed as with APZ disabled. Actual results: The page scrolls more slowly than with APZ disabled. This is particularly noticeable if you're moving the mousewheel fast, and if you're relying on the momentum from a mouse-wheel scroll.

To be clear, by speed here I'm talking about the amount by which the page's scroll offset changes in response to a given wheel gesture, not to painting or compositing speed.

Sounds like this might be the same issue as bug 1139220.

The speed of mouse wheel scrolling has improved a lot in spite of the fact that a fix for bug 1139220 has not landed yet. I'm not sure what changed.

Tested this with the bug 1139220 fix in place. The wheel scrolling speed is now comparable to non-APZ.