This should be an obvious answer, simply because we can find out from two manuals. Samus’ speed booster was first said to “Dash at supersonic speeds” in the Metroid Fusion manual , and then again in Metroid: Zero Mission.  It’s worth noting, however, that “speeds” is plural. This would suggest Samus is running faster than Mach 1.2, which would not only be the speed at which an object travels supersonic, but also the low-end. Fortunately, in Metroid: Other M, Samus generates a shock wave.
Using this shock wave, we can find out how fast Samus is running based on Mach angle. Because the shock wave has a 45° angle, this means Samus’ top speed is approximately Mach 1.41. A thinner shock wave means the object is traveling faster, and we see this when Samus shinesparks. This makes Samus’ top speed around Mach 2, which can be further supported in Super Metroid where the shinespark travels faster than the speed booster.