cubdriver's explanation is quite plausible, although he did make a typo--it should be 0.07, not 0.7.

One of the problems with supersonic airplanes is that jet engines do not like supersonic air. I won't go into the aerodynamic reasons other than to say the maximum coefficient of lift, whether that be your wing, or your compressor blades, goes down by about 50% when supersonic.

Give supersonic air to a jet engine, and they like to compressor stall. Violently. Like blades exploding through the side of the engine case.

That is why supersonic military aircraft either have a smaller intake area than the area of the compressor-face (fixed inlet jets like the T-38, F-18A or C, or F-16), or a variable-geometry inlet like the F-15, Super Hornet, or F-4.

Fixed-inlets reduce takeoff thrust, but allow supersonic flight. Rough tradeoff for a business jet.

Variable geometry inlets are heavy, complex, and maintenance-intensive. Not good for a business jet.

And, you don't "break" the sound barrier--if you are above mach, you are continuously breaking it. Drag does not go down when you go faster than Mach--it still increases, although the steepness of the curve flattens a little.

Bottom line--if the Citation X could cruise at Mach 1.05, you might save 30 minutes on a 10 hour trip--but at double the fuel cost.

So maybe you would actually have to land and refuel, thereby losing all the time you supposedly saved.

By the way, due to the huge trim changes involved when going transonic/supersonic (center of lift goes to 50% chord, on ANY airplane), you need an all-trimable tail, as most jets have. IE, the horizontal stab must move as well as the elevator. (Fighters just have a one-piece all moving slab). So, while the airframe may be slightly super capable, the limiting factor is usually the engine inlet, and overall economy of operation.