On random note, are you creating 6 faces on every single cube, even though 1-6 sides of the cube would be hidden because cubes next to it exist? If this is the case then you probably should start optimizing by removing hidden surfaces from being generated.

Considering if you have 8x8x8 blocks = 512 blocks and if each block has 6 faces that would be 3072 faces, although the only visible ones from outside would be the ones on the edge which is 8x8x6 = 384.

Edit: Quick glance on the code, you can probably reduce a lot of vertices by actually building look up table, so that you do not add same vertices coordinate multiple times, which also saves a bit.