What is a high speed stall?
When the velocity of the airflow locally exceeds speed of sound above the wing a shock wave forms and the flow detaches beyond this shock wave.



Similarly to stall, the supersonic separation of flow removes the component of lift produced by decrease of pressure on the upper surface of the wing and so the effects are similar.

I causes reduction of lift and because centre of pressure is about quarter chord on the upper surface, but midchord on the lower, it causes a significant pitch-down moment, which might be impossible to recover even if the post-stall lift is otherwise sufficient to keep the aircraft flying straight. This effect is often called Mach tuck. Supersonic planes often have all-moving elevators to have sufficient control authority to compensate for it.

A difference from normal stall is that after supersonic flow separation the lift remains proportional to angle of attack and so the aircraft continues to behave more or less normally except for the change in trim.

Mach tuck may occur as low as Mach 0.7 depending on aircraft design, because the air moves faster over the wing. It can be delayed by using swept wings, because the shock waves only form when the air velocity component perpendicular to the wing exceeds speed of sound.

Supersonic aircraft eventually encounter supersonic flow separation, but the lower surface lift is sufficient to balance the aircraft weight at that speed and altitude and the aircraft can continue flying with the upper surface flow separated.

That's not the case with most subsonic aircraft, which for efficiency reasons tend to cruise at altitudes where they have very small margin to stall. As the stall speed increases with altitude while speed of sound slightly decreases with the lower temperature there, the stall speed will eventually equal critical mach number, which creates the coffin corner and absolute ceiling for the aircraft. For most civilian transport aircraft the range between stall speed and critical mach number (where drag increases and supersonic flow separation would begin) reduces considerably at cruise altitude, but they usually don't have enough engine power to reach the actual coffin corner.