An 8-bit image uses an indexed colour palette with a maximum of 256 colours, and to make images look more realistic and colour-rich it is necessary to use patterns (fuzzy or tiled). I don't think current graphics cards are up to the extra work of generating 8-bit patterns, so it will have to be the computer's CPU that takes care of it. An extra task necessarily has to slow down the computer, regardless of whether the image takes up less memory space.