See Performance Tips.
All objects, even if not on screen, still run the logic (events, collision detection, etc.) A lot of games would be broken if this didn't happen! However rendering is usually the bottleneck, so usually it's not important that offscreen objects are still doing that.
The beginner's guide tutorial's advice to destroy objects outside the layout is more to prevent a memory leak. If every bullet you ever fire is still active in the game, the game will gradually slow down and use more and more memory.