Hmm ok I don't know what you do with the textures. My first thought was to put all of the textures for one cube in one big texture so you can add the texturecoordinates to the vertices and don't need to calculate that stuff.

Hmm ok I don't know what you do with the textures. My first thought was to put all of the textures for one cube in one big texture so you can add the texturecoordinates to vertices and don't need to calculate that stuff.