i've been getting the impression you lose some momentum or something with susp frames. into the shock .
hmmm, not so sure about that. changing direction uses up energy as objects want to go in stright lines, eitehr due to gravity or inertia.
with an FS bike, the front and rear suspension will take up a lot of the ground's irregularities so that you and all the unsprung components of the bike (which is most of the total mass) carry on unimpeded by vertical direction changes.
a useful experiment to see which is most efficient would be to take 3 bikes, rigid, front sus and full sus and roll down the same hillock from standing start and time them.... maybe....
... and according to wikipedia:
Besides providing obvious comfort to the rider, suspensions improve both safety and efficiency by helping to keep one or both wheels in contact with the ground and allowing the rider's body mass to move over the ground in a flatter trajectory.