The term 'slow to understand' can be described by two crossword solutions: DENSE (5 letters) and OBTUSE (6 letters). DENSE means having a high concentration of something, such as people or objects, in a given area. It can also refer to someone who is not very quick to understand things. OBTUSE, on the other hand, means lacking sharpness or quickness of intellect. It is often used to describe someone who is slow to understand or comprehend something.