I thought so too, but construct runs through each tile, left to right then top to bottom. Whenever the tile changes as it cycles through them, it has to do another drawcall.
So if you have two types of tiles and just keep alternating them like a checkerboard, it would create a new drawcall for each tile in the map!
And if you use multiple tilesets that are layered, you get even more drawcalls.
Animating them will slow it down even more. I've animated them in the past.
I wish scirra would optimize the tilemaps more and add more options/features for them.