Scenery Settings Effect on Performance

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?
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?