Brahmos can do S maneuver while striking target. During such maneuver, its speed is bound to be reduced significantly. If Brahmos can't start from lower speed, how will it do S maneuver? Same way, if Brahmos can regain speed from S maneuver slowdown, it can also gain speed from 0.8-0.9 Mach launch. There may be a solid booster to ignite the engine but I won't be sure about the booster having to take the speed to 2.2 Mach
The Brahmos carries a lot of secrets.