I spent a year in Pensacola, Florida attending a Navy school located on an abandoned WWII airfield known as Corry Station. The Angels were homebased at the main Naval Air Station a few miles away. They'd often practice their routines flying over Corry, and we used to watch them in sheer awe over their flying.
It's not supersonic. It's barely not supersonic, running about 700 to 720 miles per hour, but the vapor cloud forms behind the pressure wave that's building as it approaches the speed of sound. Air pressure drops quickly behind that area, condensing the water vapor. It is a result of high speed, but not a result of faster-than-sound speed.