The 'numtiles' argument in ADAPTHISTEQ refers to the number of tiles to break the entire image into, and not the actual pixel size of each tile itself. For example, if your image size was 800 x 800, then when you say
you are block processing on 100 pixels by 100 pixels sized blocks. Similarly,
would operate on 8 pixels by 8 pixels sized blocks at a time. This takes much longer, since now you have to do 10000 blocks.