usually indicates a large stiffness variation in the model say steel and say a plastic.
Use the sparse and it takes the same time regardless of the variation in stiffnesses.
It could also mean not stiff restraints.
That makes sense. For visual purposes at the end customer, I modeled a piece of equipment as a solid steel box to prevent the model from interfering with the supporting hardware so it is >> than the stiffness of the brackets. In this situation, a remote mass would normally be a better solution; however, a remote mass would be a foreign concept to a non-technical reviewer.