Zaoeyo - that's a very inefficient use of tilemaps. Since you're just using it to repeat a single image, it would be far, far more efficient to use the Tiled Background object.
If you run it in the debugger you can see it reports 441 draw rects for that tilemap, mainly because every tile is different and there are no areas of the same tile. This means it's about the same work as drawing 441 sprites. If you used a single tiled background object, it would take 1 draw call, which would be approximately 400 times more efficient.