Various forums/reviews discuss focus speed on the D600 - One of the criticisms of the camera (including Pop Photo) is that they weren't impressed with its focus speed. Some users disagree as in their experience the D600 focuses as fast as any other camera they have used, DX or FX.
I recently read about a possible speed up of focusing with the D600. The default for focus lock is set at '3' - change it to '0' or '1.' I experienced a noticeable difference, and high rate of keepers.