Why Direct3D application performs better in full screen mode?

Question

The performance of a Direct3D application seems to be significantly better in full screen mode compared to windowed mode. What are the technical reasons behind this?

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Answer 1

I'll add to @aib's answer that the rest of the screen is being managed by the OS. So, if anything else needs to be drawn/worked upon simultaneously, there has to be a performance hit.

For example, if you have a video playing in Windows Media Player in one window, then start Civilization in another, when Civ starts doing its fancy graphics, it will need to share screen space with everything else (like the video.

Whereas if the DirectX app has the full-screen, everything else might be "updating" or "playing", but not being drawn.