Indeed, simply scrolling this demo page in Chrome 9 feels very laggy. Edit: OS X, late 2009 MBP, BUT with the BetterHN tab/iframe on top (other pages are smooth though).
Scrolling was very jittery in general and page rendering/painting was also slower. Not like molasses slow but definitely not acceptable for a Core i7 machine.
With Win 7 which uses only one GPU, the Nvidia one, it is very smooth and fast as I said - definitely wouldn't want to go back.
(Is the difference between Chrome and Chromium likely to be relevant?)