Split images into smaller parts and only draw things which are marked for redrawing. Then leaving out the "cls" will also leave the screen as it was after the last flip - this would speed up the process enormously
Ok, dont bite my head off for this.But although this would seem to work, you have no garantee that the buffers will keep the flip screen in the same place all the time. That is although it seems that the backbuffer/frount buffer are always the same if you just flip, it is not always so.
Result? Well at some point the backbuffer will move, and the only thing that you will drawing on it would be the "dirty Images".