It's transferring it from video ram over the graphics bus to main memory, turning it into a pixmap, and then turning it back from a pixmap to a image, sending it all back over the graphics bus into video memory. That's not exactly a fast alternative and if anything is surely slower than:
DrawImage
GrabImage
DrawRect (over the affected area only)
That keeps it all in video ram and will be much faster than involving pixmaps.
DrawImage
GrabImage
DrawRect (over the affected area only)
That keeps it all in video ram and will be much faster than involving pixmaps.