The SR was not the fastest plane of it's day ! It was and is the fastest for the given air frame limitations and the use for which it was conceived. High speed recon and high altitude testing.

The X-15 operated in the Mach 6-7 realm but at much higher altitudes + 350K/ft where the air is thin and the aerodynamics of the air frame are practically useless. The X-15 used nose and wing tip jets to steer the craft in the upper reaches of the atomosphere.

Just my Dos Centavos !