I have experienced very similar behaviour, however the "magic event" has been different.
In some cases it has been switching between Full Screen mode to Windowed mode and then back to full screen. The gain come in the windowed mode initially, but then it is maintained when switching to full screen. Below are the steps I took to reproduce it:
1. Load X-Plane in full screen mode and open a heavy in resources aeroplane (I loaded JarDesign A320Neo)
2. Observe the frames. Mine were at around 19-20 fps
3. Load a light on resources aeroplane. I loaded the default Cessna 172
4. Notice that the frames have not improved at all
5. Change the rendering settings to windowed mode
6. Frames sky rocket.
7. Switch to "full screen". Frames remain high
8. Reload the JarDesign A320Neo. The fps now are about 34-35 (about 50% improvement)
This is in Windows 8.1, X-Plane 10.30r2, 64-bit
