Why is windowed performance so much worse than fullscreen?

[Ratchet]

On my old laptop, I preferred to play in windowed. If I were to play in fullscreen, my FPS would increase (I always assumed it had to do with rendering a portion of the screen rather than having a large bit of black rendering when playing in fullscreen.)

Now, when I play in windowed I get as much as 3x the FPS. If VSync is enabled, my FPS will only go up to 60 FPS in both windowed and fullscreen because that is what the refresh rate is of my TV. If I disable VSync, my FPS gets about 400 (1360x768) in Fullscreen and around 1000 in windowed (800x600)