My understanding is that your entire tilemap bitmap is loaded and stored in memory, if it is on the layout, regardless if the tiles are used or not. Rendering wise if they're invisible, you're not drawing them, and thus should take no additional resources. If it drawn, you'll have your usual fill rate considerations (which normally wouldn't need to be considered at all, unless you're putting layers of layers of stuff with force own texture stacked on top of each other within a single viewport for some reason). Similarly, tiles outside the viewport should not be drawn as well for obvious reasons.