We have been developing a volumetric computing graphics cluster system in the form of a PC cluster with its rendering and calculation performance enhanced by graphics boards and dedicated devices. The goal of the system is to perform real-time simulation for a practical surgical simulator. A space-partition scheme for parallel processing inevitably requires data communications for both simulation and visualization. We studied the effects of both types of communications through experiments. On the basis of the results, we discuss a performance model and propose a performance metric for time-restricted processing. To provide an example, we evaluated our VGCluster system by using the proposed metric. The metric shows the effect of sustaining scalability by using a dedicated image-composition device.