Why is it that frame rates drop with multiple windows open?

Hi all:

haven´t been around for a while, so I´m no more up to date with latest enhancements in this field.
I was discussing a 3 beamer solution in a german homecockpit builder forum. My inherent problem is, that only the best PC around will just suffice to meet my frame rate expectations when using a multiple window surround (180° +) view (in combination with TH2G and Immersive display lite 2.0).
My question: Why is it that the number of windows in Microsoft´s FS is so "depressive" to the frame rate? Has it to do with the source code (which obviously nobody knows)? Can anybody explain this to me in layman words? Why is there no "go-around" for this problem?
Why exactly is FS so dependent on a good CPU (and GPU)?
Is it the same with other platforms such as X-plane?

Looking forward to you inputs...

Regards
Chris