Tiff is a sort of meta-fileformat, it can support any colorspace, any compression and custumn meta tags. So you could have a scanner that likes outputting images with 1 bit for black-white and 13bits for shades of purple ... all compressed with let's say the rar-algorithm.

So working with tiff really is no fun. Qt4 works with 32bit ARGB internally and as far as I know cannot be told to work with other stuff. blowing up 1bit image data to 32bit looks like a waste of space. So if you need fast processing of lot's of images, you might be faster with your own implementation.