Curious about GPU vs CPU

[Johan_Dees 84]

Imagine, I have a middel setting, in whatever flightsim.

It gives me 25 fps.

 

The CPU is working hard, but the GPU is waiting for the next image to project to the monitor. Under used. 25 fps

 

Now, the GPU can do much more, and doing shaders, or anti aliassing for free, since its waiting and waiting. 25 fps.

 

Then enabled, I see th fps drop.. why ? why does the CPU slow down in putting out frames (the same it was doing before) and

the GPU doing its extra thing cause an effect on the CPU ? 15 fps. ?

 

Puzzled here.. any one with knowledge can shine a light on this behaviour ?

[Johan_Dees 84]

Radiocheck ?