Some games run faster on my system if running in a window, FSX does as well.
SABOW _always_ running in window too. If you not disable this in options.
From what you have indicated my understanding of running in a window may not be correct. I have always taken it as being running in a seperate window, and that is what I thought the option was.
For many years, there is no difference when running in a window or full screen mode.
But in full screen mode there are certain problems when starting/closing the program. Therefore, in the game it is practically not used.