I recently starting using
gdal2tiles.py (and its parallel implementation) to generate tiled images from raster datasets for kml interpretation and I'm shocked by how computationally intensive the process is. For example, rendering a 3.8MB PNG image of the US (35000 x 15000 px - or 2500 dpi @ 14" x 6") with tile levels 0-7 takes over an hour on a modern quad-core system. Even then, the resolution isn't all that great!
What is happening during the tiling process that is so computationally intensive?