Actually, this time I do not know if the meaning is actually correct. The name sometimes appears as rAtula instead of rAhula, but neither seems to clearly associate with efficiency. In either case, the root is likely to be either related to rabh/labh/grah which means to seize or gain, or to rah which means to separate. Of course, since it does not appear till Buddhist literature, it may not be Sanskrit at all (i.e. it could be some evolved form): for example, for example it can be related to words for red derived from rakta, blood, derived in turn from ranj, to color.
In addition, one should not forget the great seizer rAhu, astronomically one of the points of intersection of the moons orbit with the plane of earth's orbit thus sitting with the moon during eclipses and responsible for a lot of misfortune according to Indian astrology, starting already in the vedic period. The Buddhist sources often conflate the concepts of demons (fighters against truth) with rAhu.
I would be interested in knowing where the meaning efficient came from.