* Geographic Plain: A large, flat area of land with few changes in elevation. This definition doesn't relate to speed.
* Aircraft: You might be thinking of a "plane" as in an airplane. In this case, "fastest" would refer to the airplane's speed. The SR-71 Blackbird holds the world record for the fastest manned aircraft, reaching speeds of over Mach 3.3 (over 2,190 mph).
* A flat surface: This could refer to something like a table or a floor. It doesn't have speed in the context of a flat surface.
If you could clarify what you mean by "fastest plain", I can give you a more accurate answer.
