Why such a difference

[djbozz]

I been flight simming for years & only just noticed this, I have always run in windowed mode for access to my maps, MCE & plan-g etc, I normally get 15 - 20 fps, I managed to switch to full screen by coughing (copilot is hard of hearing), my fps shot up to 30fps, where I have fsx set to, my question is why?

[Daube]

Because the video cards are optimized for running in full screen.
Also, when you are in windowed mode, the card has to draw the game AND the desktop items at the same time, so it has more work to do.