1) Set video card to 5760x1080 eyefinity mode
2) Start Arma, set resolution to 5760x1080, full-screen.
3) Enter editor, place man on a map, preview
4) Go to video settings - observe frame rate (I was getting ~64fps at this point)
5) Alt-tab back to the Windows desktop
6) Switch back to Arma. Observe that the frame rate is now much lower (33fps, in my case).