If you want to play DayZ in windowed fullscreen without borders (allowing you to switch between apps without delay, or clicking on your other monitor without delay or the game minimising) here is what to do:
Edit your file DayZ.cfg that is located in “My documentsDayZ”, and change the following lines:
Render_W=1920; (width of the image to render)
Render_H=1080; (height of the image to render)
Resolution_W=1920; (width of the image to render in windowed mode)
Resolution_H=1080; (height of the image to render in windowed mode)
winX=0; (position X of the window when the game starts – will not apply if you want to play in fullscreen windowed)
winY=0; (position Y of the window when the game starts – will not apply if you want to play in fullscreen windowed)
winW=1920; (width of the window)
winH=1080; (height of the window)
Windowed=1; (activates windowed mode)
WindowedMode=3; (the magic parameter! 1=normal window, custom size, 2=maximised window with border, 3=fullscreen window without border!)
Note: if the game loses focus (if you alt tab or switch to another app, the sound will stop and the image will be fixed, it is probably wanted by the devs to avoid hardcore campers with snipers who would just sit waiting all day long)
I could not notice any performence change by playing in fullscreen windowed mode.
I hope that helps!