You might have seen it in Noitu Love 2, that awesome wall in the background that makes the scene look 3D.
Here is one example of how it can be done no extensions or special objects or anything. Also, it is technically done in only one event. There's probably other ways too.
Note: you can't do the floor or roof with this example, so you'd have to make it either solid color or do it like Noitu Love: the ground is on the level with horizon.
(980 kb file)
I don't know if it's slow because I don't have another computer for testing.