Why multiple monitors = less FPS in FS X?

Hi,I do not understand why , less in FS 9 and more in FS X, a config with multiple monitors and "undocking" windows like 2d cockpit and Radio = less FPS !!When I fly without undocking = 24 FPS (average) with ASv 6.5 and WOAI and most of the sliders to the middle. When "undocking = 9 FPS !!Any idea?I searched on Google but did not find a response.Thanks in advance.Have a nice day.