I am trying to decide if it is better to use a pre-rendered large image for a scrolling map game or to render the tiles individual on screen each frame.  I have tried to program the game both ways and don't see any obvious difference in speed, but that might be due to my lack of experiences. 
Besides for memory, is there a speed reasons to not use a pre-rendered map?
Memory and speed are closely related.  If your tile set fits in video memory, but the pre-rendered map doesn't, speed will suffer.
Pygame: Tiled Map or Large Image