lower BGM volume
Pre-render layers for better performance

Pre-render grass and trees game tiles, since they are dense layers. This seems to speed up the game by 5-10 fps at the cost of 10% more browser memory (at least on area01).
Remove unnecessary call to me.game.world.collide

From the FAQ (https://github.com/melonjs/melonJS/wiki/Frequently-Asked-Questions): "Avoid calling me.game.world.collide() in every entity! This function is very slow, and careful consideration should go into which entities call it. For most games, the main player should be the only entity calling this function, with other entities responding to the collisions in their own onCollision callback. "