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?
Depends on the size of the map you want to make, however, with the actual technologies it's very hard to see a tile-map "rendered" to take longer than expected, tiled based games are almost extinguished, however is always a good practice and a starting point to the world of game programming
Pygame: Tiled Map or Large Image