The Plane speed factor patch, located in Advanced settings > vehicles controls the speed that planes move following from the plane speed displayed in the game. The plane speed displayed will always be the same no matter the setting.
The available settings are 1/4 (default), 1/3, 1/2 and 1/1.
Planes in OpenTTD by default move across the game world at a quarter of the displayed speed. This allows planes to be balanced in the game's economy and gives planes a visual sense of elevation over the ground in flight.