Why do people have different levels of intellect or can understand things easier and quicker?

Just the way it is. It is so for a reason. so that people who understand life can communicate without upsetting those who don't because such an upset is unnecessary to the divine plan. They cannot help it or do anything about it. That does not make them lesser beings they have no choice in it. They are the innocent and god loves them all. They may also be the fortunate.