Pretty good. It might lag if someone holds in spacebar for a while and therefore throwing like 500 snowballs(which is something you should fix), but to answer your question, no, it's not laggy. The snowball physics are bad though, they bounce off of walls and fly all around the place, try to work that out. Also, possibly make them dissolve when they hit an object or surface. Like others are saying, put some sort of outline around the character, and work on his animations more. I guess the graphics aren't the worst thing in the world, but they're just really bland. Even if you're not the best artist, try to add at least some more color and detail to it. Just work on that stuff, and it seems like it'll be quite a fun game.