Normally, if two points of view are at odds, then they are argued out. With parallel thinking, both points of view are laid out alongside each other. Later on, if it is essential to decide between the two, a decision is made. So there is not argument at every step. Perhaps the biggest obstacle to quick and effective thinking is the ego.
People tend to use thinking to parade their egos.
Thinking is used to attack and put down other people. Thinking is used to get your own way. Thinking is used to show others how clever you are. Thinking is used to express personal antagonisms. Someone will choose to disagree on a point simply to show up the person who has made that point.