The return passage of the 12th Planet is dictated by a combination of forces - momentum, attraction, and (for want of a more scientific term to describe this process) the call of the wild. The 12th Planet has stopped, and hovers for some years before beginning its return passage. The rate of startup essentially mirrors the rate of stoppage, so movement is slow at first, almost imperceptible.
During its first passage the 12th Planet had been moving already, and the factor of momentum allowed it to accelerate an already existing rate of speed when boring in on the Solar System. Where space does not have much matter to create resistance to motion, momentum is in and of itself a factor. Why does a ball in a track on a straight-away continue to roll after the initial push? What keeps it rolling? Momentum, which in fact is an interplay between the attraction and repulsion forces of bodies around the object in motion, is a factor in its own right. Momentum is what causes the planets to orbit, perpetually, and the Earth to rotate, perpetually. Thus, the 12th Planet's return requires that it again build momentum, which it does not do to a comparable degree until it is inside the Solar System again and well into the net of gravitational attraction that the Sun presents. The return passage thus finds the 12th Planet more sedate in its approach but nearly equivalent in the speed of its passage through the Solar System, which is dictated more by the relative size of the Sun and the 12th Planet than any other factors.
The 12th Planet hovers for the length of time it does before returning because of what we will term the call of the wild. Having stopped in its tracks the 12th Planet is in a vulnerable position, and in point of fact could become caught in a new pattern of motion should the objects around it present a new dynamic. It has stopped, dead still, and thus is in a virginal position of having no commitments. During most passages of the 12th Planet there is no contest, but in some cases there are other attractions nearby that create confusion. The upshot of this is that the 12th Planet may delay longer before setting out on its return passage, but the factors in your part of the Universe are not such as to change the outcome.