I believe that psychology is one of the most important parts of design. The very basis of designing a successful system or product relies on audiences gaining something from it. Because audiences are a primary focus, the way audiences think should always be taken into consideration. This allows for conventions and knowledge to go into a design that will help with the designer’s intentions.
Performance load is the perfect example of this. When people are faced with problems in a system that takes too long to be worth the effort, they give up on it. However if a designer were to make this system easy a person is much more likely to use it. This is just scratching the surface of psychology in design, but things that appeal to the human mind will always be used over things that don’t.