Hello Lanebobane, welcome to SF!
When you connect a second screen you have the option of "cloning" your primary screen to it or "extending your desktop". Or, sometimes, neither. It seems you've set it to clone. Since you did not post any lt details it is difficult to give examples...i wonder what it is that you do to fix it now, as i would expect that to be the sam screen where you can set this right...
Also, dending on mobo, on board graphics, if your system is new enough to run Windows 7, I would be very surprised if running lt screen AND external, caused a noticeable processing power diff?