That got your attention, eh? Sorry, this is not a tip on how to tune your X-Plane system; it's a tip for aircraft authors to make sure their 3-d cockpits are running at maximum performance.
Prefill is when X-Plane blocks out the clouds that will be behind the airplane cockpit. The biggest cause of GPU slow-down is cloud rendering, so reducing the area that the clouds have to fill is really important.
In the 2-d cockpit, X-Plane pre-fills automatically. But in a 3-d cockpit, the airplane author has to specify which objects should be used to pre-fill.