Physics engines are like magic and it is pretty hard understand how the work even if you read the paper many times. Also physics engines are interesting not only because four the dynamics, but also for the way constraints are solved. There are many other problems were you could apply this same techniques. This JavaScript engine it's a couple of pages long, it's easy to understand and potentially could give you a new vision on how to solve problems. The reason why is uses do much CPU time is because the collision detection algorithm compares every single pair of objects, to speed up that part I'd need to add what is called a 'broad phase' but that would complicate the code and the point was to write clear code and leave the low hanging fruit for the reader.