The planned MG procedure for 3-D works Bottom Up. This is the Top.

The final grid :     1 grid   33 x 33 x 64    4th generation
Grid coarsening:     8 grids  17 x 17 x 32    3rd generation
                    64 grids   9 x  9 x 16    2nd generation
                   512 grids   5 x  5 x  8    1st generation

The number of elements is approximately the same for each generation.
Thus the FEA procedure will take 4 times as long as for a single grid.

Because FEA is so time-consuming, it cannot be compensated by shorter
iterations in the Solving phase, where MultiGrid would show its power.