Creating tiles from a single large map document is very computation hungry, because the large and many polygons it has. For each small tile(anything up from zoom level 5 counts as small) the clipping algorithm needs to iterate through a large polygon with thousands of nodes. This sounds unnecessary ifwe know that the tile only contains just 10 or so nodes from that large polygon. A good way to avoid this is to use the system memory to counter the need for proccessing power. T
That being said, if we want to generate level 5 tiles then we first want to store the level 3 tiles in memory and use them. We can then cut each of those tiles into 16 pieces(not 4, because we only use odd zoom levels) so in the end we will only iterate on a fraction of the original map document to generate tiles.
This will give us no more large polygons with thousands of hundreds of nodes, so in the end less time to generate the wanted files.