Speed wins out over power while this mode is active, as the archer fires more rapidly, reducing aim animation by (5*[Dex-10])/100 s (max: -2.0s) but without any chance of inflicting regular critical hits. Master Archer reduces aim animation by another -1.0s (this additional bonus can allow Rapid Shot to reach -3.0s aim animation length reduction).
Aim speed can only be reduced to 0. Most items with the Rapid Aim property reduce the aim time by 0.3s. For longbows and shortbows (aim time of 0.3s and 0.2s respectively) Rapid Shot is therefore useless if one of those items is equipped.
Stealth, Pinpoint Strike and Death Hex can force a critical hit even if rapid shot is on. It will convert the regular attacks into criticals and will misses into regular attacks. Basically, the lower your ranged critical chance % the higher chance you have of forcing criticals.