What is decalage in airplanes and how important is it in respect to airplane performance?
This article from flying magazine has a nice definition (including some history)
Decalage — a French word meaning “shift” or “offset” — is, broadly speaking, a difference between the incidences of any two lifting surfaces. It was originally applied to the two wings of a biplane: In the usual arrangement, the upper wing was farther forward than the lower and had a larger (can’t get away from that term!) angle of incidence — called positive decalage — so that it stalled first, shifting the center of lift aft and providing an automatic nose-down moment for recovery.
In a monoplane, the term refers to the angles at which the wing and the stabilizer (or canard) are attached to the fuselage. For an airplane to be longitudinally stable, it must have positive aerodynamic decalage; roughly speaking, the forward surface must be at a greater angle of attack than the aft one. This principle applies to conventional airplanes and to canards alike.