Keep in mind that windowed mode is designated non-exclusive to your video game. For most people on avg graphic settings in a game it means nothing, though if you are trying to get the highest graphic quality your computer can muster you will see reduced performance while running windowed.
It's always been a irritation of mine the non-exclusive thing as I run a 30in and a 24 in on the side. You'd think with the technology we have on the hardware end these days the software engineers can find a way to reduce or eliminate the windowed performance factor. Especially since in most games I can still view the items in the second screen, hardware monitors/chat clients etc, I just can't use them untill I alt tab to minimize the game.
Most games that run in windowed mode are seemless between screens, so using the second screen all you have to do is move the mouse curser over to the other screen.