I want to understand what others say immediately.
So, I searched for related document, and I found a nice article. Here.
The reason why you have to spend so much time to understand some information is just you have too much stress on your working memory.
Working memory is a type of information storing system, and it is a temporal storage of information.
Working memory has 3 characteristics.
1. It can handle up to 3~7 information.
2. It concentrates on onlly neessary information.
3. It delete irrelevant information one after another.
Based on these knowledge, the author explains the reason, like this.
When we cannot understand something quickly, It may be due to the attention to the irrelevant information that occupy your limited working memory.
And those who care about irrevant information tend to become so anxious that they always check whether they miss some information. (I agree with their opinion that parsonality has some impact on the capability of memorizing.)
And in that article, the author recommends to write diary or memo, and do meditation to reduce stress, so as to be "quick understander"