Hi Finn. The fastest airplane I flew was a small jet called G91. Its top speed was Mach 0.92.
The Number of Mach is one of the ways used in fast jets to measure speed. 1 Mach is equal to the speed of sound, so my airplane was capable of flying just below the speed of sound.
If you have any other questions, do not hesitate to ask them!!
Have a nice week.