Hmmm, so.
With a normal bg, zoom-out often stutters if I ended the mouseover state by moving to the side of the dock, rather than above. This only happens occasionally, and not as severely, with a tiled bg.
With a tiled bg, zooming in on icons, alternates between stuttering and fine, seemingly at random. I'm set to 'performance'.
With any bg, when set to ghost mode, zoom-in always stutters.
With a tiled bg, ghost mode gets these funny artefacts when I zoom in:

Sometimes I even get chopped, repeated versions of the tile with the icon.