Transonic flight occurs in the range of Mach 0.72 to 1.0 (965–1,235 km/h (600–767 mph) at sea level). At this speed we see a rapid increase in drag from about Mach 0.8, and at the same time the fuel costs rise dramatically. Attempts to reduce wave drag can be seen on all high-speed aircraft. Most notable is the use of swept wings, but another common form is a wasp-waist fuselage as a side effect of the Whitcomb area rule.
Severe instability can occur at transonic speeds. Shock waves move through the air at the speed of sound. When an object such as an aircraft also moves at the speed of sound, these shock waves build up in front of it to form a single, very large shock wave. During transonic flight, the plane must pass through this large shock wave, as well as contend with the instability caused by air moving faster than sound over parts of the wing and slower in other parts.

Why Planes Don’t Fly Faster
Shock Wave Formation in Transonic Flight