My own experience is that if I just want to gain a basic understanding of a result, then I do not struggle to understand all the details of the paper. I read it much as a might a news story.
On the other hand, if I want to use the result, or use the approach in solving a different problem, then I'm tenacious, and sometimes very very slow. I am currently reading a paper where I have gotten stuck in my understanding half way through the second of 5 pages. It's been that way for a while despite my efforts. I understand the overall result; no problem there. But since I want to use the result and method in a piece of my own work, I really need to overcome the barrier that is confronting me ... and in my case, I suspect it will only be by enlisting the help of a colleague who can explain it to me.
Ultimately, I might have to read another paper that gives me sufficient background, before coming back to this one. I imagine, but have not evidence, that my approach is not unlike that of other people.