They have up to three racks for a flight simulator? I would've never thought they're that complex. Do they need that much computing power or is it other devices that are needed?
In general aside from the boxes handling the graphics projection, most of the 'compute' is idle a lot of the time, even when the simulator is running. There's usually a big burst of work to prep and load the simulation into the simulator, then minimal (but very latency sensitive) chatter back and forth between the various simulator components and the backend compute as the simulation runs.