Pretty sure the lag these posters are referring to is just the DriveBYWire system. They're known for having a lag. Although i find it tends to make for extremely smooth take off if you don't romp on the gas. I've driven several Drive By Wire systems, a bunch of BMW's, Cadillac CTS, and my work truck 2010 GMC 2500HD, and they all experience this slight lag upon take off. It goes away if you really step on it, but if you just do a normal take off its a noticeable lag but makes for extremely smooth and gentle take off.
Call me crazy, but i thought i once read that was the purpose of this lag, for a smooth gradual take off. The computer for some reason puts the throttle on gradual. I know that some tunes remove this "feature" so its back to the normal way.