[Lithophantastic]

Hi, I'm planning to purchase a new Rockler Shark CNC machine. I currently use larger kit built machine with photoVCarve and Mach 3 for processing. The Rockler machine uses PhotoVCarve and it's own processor. Before purchasing the machine, I asked the shop owner to run a lithophane on the Rockler Shark displayed in his shop. I provided the new bit and the processed gcode on a thumb drive. The estimated carve time was 6 to 7 hours for a 16" X 20" lithophane.
Imagine my surprise when I get a call from the shop owner and he tells me it took 16 hours. Plus, there is a 1/4" raised line running straight through the left side of the acrylic. He saw it and strongly believes it is not a problem with the machine. He guesses it is the software. I'm not sure if that means PhotoVCarve software or the Rockler processing interface.
This shop owner has an excellent reputation and I trust him.
I've seen this raised line problem before with my current machine and set up. Although, it's usually in the form of a skip of a single line. Drives me crazy because it ruins the whole piece.
What do you think?
First, why did it run so long?
Second, is this a machine error?
90% of our machine time is for carving lithophanes.
Thanks!