All game assests are broken into portions according to the game Levels. So while user play Level #1, next level is being loaded in background.
But there is a strange behaviour with the sprites when preloading is active. Sprites are played way faster.
Here is a test case: http://cjstest.comlu.com/index.html
- Click "Play" button. You can see a moving rect at 16 FPS speed.
- Click "Load L2" button. You'll notice that rect has started moving faster.