Another one with the problem here, all fps measurements in Main Menu and with nVidia Shadowplay fps-Overlay, on my machine the game runs with VSync activated at stable 60 fps, ALT+TAB puts the game in windowed mode and reduces the fps to a stable 40, ALT+ENTER brings the game back to full screen and halves the fps to stable 20 fps, ALT+TAB again puts the game in windowed mode running at stable 18 fps, ALT+ENTER brings it back to full screen running at stable 12 fps, ALT+TAB windowed mode 10 fps, you see where this is going.
If you need further information, fell free to ask!
