1. a single image will be quicker than having it split up, because everytime a tile is changed, the renderer has to make another draw call..
here's a quote:
"It is not appropriate to use the Tilemap object to display large images. The tilemap object draws tile-by-tile, but can optimise if there are areas of the same tile. This is described in the blog post Tilemap tidbits. If you paste a large image in to the Tilemap object, and then stamp a grid of tiles to display the large image, every tile is different. Therefore no optimisation is possible, and a draw call must be issued for each tile. If the image needs a 10x10 grid of tiles to display, then it will need to issue 100 draw calls. A single sprite or Tiled Background, by comparison, takes a single draw call each. Therefore this type of use of the Tilemap object can be hundreds of times less efficient than using another object. The Tilemap object should only be used for tile based games, where a single tile is a useful level design block by itself, rather than part of a larger image."
2. same issue- if you have multiple tilemaps layered over each other, that hadds to the number of draw calls- so it will slow the game down.
pretty unfortunate, I know.