The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
23. (The soul passes through the stages of its descent) in a not very long time; on account of the special statement.
A doubt arises with reference to the period beginning with the soul's becoming ether and extending up to its entering into rice, &c., viz. whether the soul remains a long time in the state of similarity to each of the stages of its way before it enters into similarity to the next one, or only a short time.--The pûrvapakshin maintains that, on account of the absence of a definite text, no binding rule exists.--To this we reply that the souls remain in the state of similarity to ether. &c., for a short period only before they fall to the earth in raindrops. We infer this from the circumstance of the text making a special statement. For after having said that the souls enter into rice, &c., it adds, 'From thence the escape is beset with more pain;' a statement implying that the escape from the previous states was comparatively easy and pleasant. Now this difference in point of pleasantness must be based on the comparative shortness or length of the escape; for as, at that time, the body is not yet formed, enjoyment (in the ordinary sense) is not possible. Hence we conclude that, up to the moment when the souls enter into rice, &c., their descent is accomplished in a short time.