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How to quickly understand the main idea when reading a paper?

I have problems in reading papers, specifically papers where the subject is not so new to me and I am familiar with the problem which the writer wants to solve.

How to find the those parts which talk about the main idea?

When I go to their suggested work to understand the main idea, I have to have first read their previous sections, because they define some notations and definition which they use them on their suggested work. When I go to read those parts to understand the notation I tell my self is work ok? because after reading the notations and definitions , I should start to learn their suggested method. and also that one still have its challenges.

My advisor said you should understand the main idea of a paper in one hour, or at maximum in two hours. But I need at least 5 hours.

(Maybe help: I am reading a paper in the field of computer networks.)