Scenery Settings Effect on Performance

[Bubblehead]

Two questions:
In the "Select Airport" window at the bottom, there are two select dots.
One states "Search Default Scenery" and the other "Search Add-on Scenery". Which one should I select?

Also I have lots of add-on sceneries which I downloaded and installed including the USA 10M Terrain Mesh, Mega Scenery and many others all of which are enabled. What effect does scenery have on overall performance performance?

[DazzleStar]

http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1226348700