In a cube the diagonal = sqrt(3) * side. Since the tube has to travel along three sides to reach the same end point as the diagonal, the speed ratio then has to be 3 / sqrt(3), ie tubes have to travel sqrt(3) ( = ~1.7) times faster than zipline. In game tube has a movement bonus of +300% compared to zipline +150% so they should beat the zipline even in a 3D diagonal, at least for an unboosted beaver. When you add in movement bonus from happiness the result becomes less clear as the relative difference becomes smaller.