@retrodude - this is helpful, thanks.
I feel like I'm already doing this, though. I have 2 tiled backgrounds of 1 solid color each (64x64), with 3-4 sprites that have 5-6 unanimated frames for decoration (max size is 306 x 202). The rest are enemies & small ui.
My level size is pretty huge - that I won't argue with (20,000 x 20,0000) but originally I was told that it only processes what is currently on screen. Does this then mean it doesn't necessarily matter whats on-screen, because it will affect the game regardless?